ANSI/NCSL Z540.3:2006
Transcription
ANSI/NCSL Z540.3:2006
December 2006 measure NCSL INTERNATIONAL The Journal of Measurement Science Vol. 1 No. 4 • December 2006 NCSL International In This Issue: measure • The Journal of Measurement Science Uncertainties Related to Thermal Expansion in Dimensional Metrology Gravimetric Calibration of Volumetric Standards with Capacities Exceeding Five Gallons A Theory for RF and Microwave Scalar Reflectometer Errors ANSI/NCSL Z540.3:2006: Requirements for the Calibration of Measuring and Test Equipment Vol. 1 No. 4 measure N C S L I N T E R N AT I O N A L T h e J o u r n a l o f M e a s u re m e n t S c i e n c e WELCOME to NCSLI measure, a metrology journal published by NCSL International (NCSLI), for the benefit of its membership. Contents Features 22 2 0 0 7 N C S L I Wo r k s h o p & Sy mpos ium Vol. 1 No. 4 • December 2006 26 See Page 19 32 S P E C I A L R E P O RTS T he C I P M Wo r k i n g G ro u p o n M e t ro l o g y o f M a t e r i a l s Seton Bennett and Graham Sims AN S I/ N C SL Z 5 40 . 3: 2 0 06 : Re qu ir e m e n t s f o r t h e C a l i b r a t i o n o f M e a s u r i n g a nd Te s t E q u i p m e n t Del Caldwell T E C H N I C A L PA P ER S Unc er tainties Re late d to T her mal Exp ans ion i n D i m e n s i o n a l M e t ro l o g y Ted Doiron 38 A T h e o r y f o r R F a n d M i c ro w a v e S c a l a r R e f l e c t o m e t e r E r r o r s Robert D. Moyer 46 A D i re c t C o m p a r i s o n S y st e m f o r M e a su r i n g R a d i o F re q ue nc y P o we r ( 100 kH z to 18 G Hz ) Ronald Ginley 50 Remo te Tim e C alibratio ns vi a t h e N IS T T i m e M e a s ure m e n t a n d A n a l y si s S e r v ic e Michael A. Lombardi and Andrew N. Novick 60 R E V I E W PA P ER S G r a v i m e t r i c C a l i b r a t i o n o f Vo l u m e t r i c S t a n d a rd s w it h Cap ac it i es E x c eed in g F i v e G al l o ns L.F. Eason Departments CONTACT NCSLI 3 L e t t e r f ro m t h e E d i t o r B u s i n e s s O ff i c e : Craig Gulka, Business Manager NCSL International 2995 Wilderness Place, Suite 107 Boulder, CO 80301-5404 USA Phone: 303-440-3339 Fax: 303-440-3384 Email: info@ncsli.org 3 Le tt ers 5 I n t e r na t i o n al NM I Ne w s 12 M e t ro l o g y N e w s 75 N e w P ro d u c t s 79 A dv e rt i s er In d e x 80 Cla ssifie ds NCSLI measure I n f o r m a t i o n : www.ncsli.org/measure/ Vol. 1 No. 4 • December 2006 MEASURE | 1 NCSLI Member Benefit in the Spotlight P u b l i c a t i o n s a n d Vi d e o s NCSL International has developed an extensive library of publications and training videos to educate NCSL International members about various areas in metrology, including: International standards; laboratory procedures; measurement practices; and current metrology services and seminars. Some of the publications available through NCSL International include: • ANS/ISO/IEC 17025:2005 • ANSI/NCSL Z540-1-1994 (R2002) Standard • ANSI/NCSL Z540-1-1994 Handbook • ANSI/NCSL Z540-2-1997 (R2002) "U.S. Guide to the Expression of Uncertainty in Measurement" • Recommended Practices (RPs) Calibration Intervals; Laboratory Design; Laboratory Environments; Interlaboratory Comparisons; Determining and Reporting Measurement Uncertainties; Calibration Procedures; and more. • Recommended Intrinsic/Derived Standards Practices (RISPs) Array Josephson Junction; Quantized Hall Resistance; Deadweight Pressure Gauges; and Two-Pressure, TwoTemperature Humidity Generator. • Laboratory Management Acronyms and Abbreviations; Glossary of MetrologyRelated Terms; Guide to Achieving Laboratory Accreditation; Calibration Laboratory Manager's Handbook; Comparison Between ANSI/ISO/IEC 17025:2000 and 17025:2005; Benchmarking Survey Results; and more. measure N C S L I N T E RN AT I O N A L T h e J o u r n a l o f M e a s u re m e n t S c i e n c e NCSLI measure (ISSN #19315775) is a metrology journal published by NCSL International (NCSLI). The journal's primary audience is calibration laboratory personnel, from laboratory managers to project leaders to technicians. measure provides NCSLI members with practical and up-to-date information on calibration techniques, uncertainty analysis, measurement standards, laboratory accreditation, and quality processes, as well as providing timely metrology review articles. Each issue will contain technically reviewed metrology articles, new products/services from NCSLI member organizations, technical tips, national metrology institute news, and other metrology information. Information for potential authors, including paper format, copyright form, and a description of the review process is available at www.ncsli.org/measure/ami.cfm. Information on contributing Technical Tips, new product/service submission, and letters to the editor is available at www.ncsli.org/measure/tc.cfm. Advertising information is available at www.ncsli.org/measure/ads.cfm. Managing Editor Richard B. Pettit, Sandia National Laboratories (Retired), 7808 Hendrix, NE, Albuquerque, NM 87110 USA. Email: randepettit@comcast.net NMI/Metrology News Editor: Michael Lombardi, NIST, Mailcode 847.00, 325 Broadway, Boulder, CO 80305-3328 USA. Email: lombardi@nist.gov New Product/Service Announcements: NCSLI Business Office, 2995 Wilderness Place, Suite 107, Boulder, CO 80301-5404 USA. Email: info@ncsli.org Technical Support Team: Norman Belecki, Retired, 7413 Mill Run Dr., Derwood, MD 20855-1156. Belinda Collins, National Institute of Standards and Technology (NIST), USA Salvador Echeverria, Centro Nacional de Metrologia (CENAM), Mexico Andy Henson, National Physical Laboratory (NPL), United Kingdom Klaus Jaeger, Jaeger Enterprises, USA Dianne Lalla-Rodrigues, Antigua and Barbuda Bureau of Standards, Antigua and Barbuda Angela Samuel, National Measurement Institute (NMI), Australia Klaus-Deter Sommer, Landesamt fuer Mess und Eichwesen Thueringen (LMET), Germany • Organization listed in the Directory of Standards Laboratories (Web Site) Alan Steele, National Research Council (NRC), Canada • Post Job Listings in the NCSLI Jobs database Andrew Wallard, Bureau International des Poids et Mesures (BIPM), France Pete Unger, American Association for Laboratory Accreditation (A2LA), USA • Training Information Directory Tom Wunsch, Sandia National Laboratories (SNL), USA • NCSL International MEASURE Journal and Quarterly Newsletter Production Editor: Mary Sweet, Sweet Design, Boulder, CO 80304 USA Email: msweet@boulder.net • Training Video Tapes For more information about membership benefits, contact the NCSLI Business Office at 2995 Wilderness Place, Suite 107, Boulder, CO 80301 USA (Phone: 303-440-3339) or visit the web site at www.ncsli.org/memberships/ 2 | MEASURE Copyright © 2006, NCSL International. Permission to quote excerpts or to reprint any figures or tables should be obtained directly from an author. NCSL International, for its part, hereby grants permission to quote excerpts and reprint figures and/or tables from this journal with acknowledgment of the source. Individual teachers, students, researchers, and libraries in nonprofit institutions and acting for them are permitted to make hard copies of articles for use in teaching or research, provided such copies are not sold. Copying of articles for sale by document delivery services or suppliers, or beyond the free copying allowed above, is not permitted. Reproduction in a reprint collection, or for advertising or promotional purposes, or republication in any form requires permission of one of the authors and written permission from NCSL International. www.ncsli.org Letter from the Editor Letters This is the fourth and final issue of NCSLI measure for 2006. It has been a very enjoyable and educational experience for me, and I have learned a lot about the electronic publishing business. Each issue has surpassed my high expectations, and I am very pleased with the positive response we have received from the NCSLI membership. Again, I would like to thank the NCSLI Board of Directors, and especially Tom Wunsch, of Sandia National Laboratories, who conceived of the journal format and has continued to provide ideas and leadership for each issue. As I think about 2007, my vision is to expand the journal from the current 80 pages to 96 pages. Most of that expansion will include more Technical Papers, as well as Special Reports, Review Papers, and Technical Tips. Currently we are averaging about eight papers per issue. With the expanded journal, we will be able to publish about ten papers per issue. In addition, we will have more space for NCSLI member organization new product/service announcements and for paid advertisements. Recently, measure has received several technical papers from international sources, including Canada, Egypt, Japan, and Chile. This is strong evidence that our journal is both being recognized as a valuable publication resource in metrology, and that it is obtaining international recognition. As part of my NCSLI involvement as Vice-President of Measurement Science and Technology, I have been working with Jim Wheeler, of the U.S. Navy in his role as the chair of the Measurement Comparisons Programs Committee to request that every NCSLI sponsored interlaboratory comparison (ILC) results in an article in measure. In this way, the important results and lessons learned from each NCSLI ILC will become part of our permanent record and will better benefit all NCSLI members. I also want to acknowledge a special thanks to Michael Lombardi, NIST, who has collected and edited the NMI and Metrology News items. Mike has also contributed several excellent technical papers and review articles documenting the important services of the Time and Frequency Division at NIST, Boulder. He has also provided very valuable editorial suggestions and assistance as we strive to develop our own style and processes for the journal. Mike has a strong interest and educational background in technical writing – and they have served us well! His ideas and support are greatly appreciated. Finally, the NCSLI Business Office is in the process of developing an online database with information on each paper that is submitted for publication, the long list of technical reviewers who provide the valuable service of reading and commenting on each paper, new product/service announcements, NCSLI member advertisers, etc. When completed, this new tool will streamline the multitude of tasks that need to be completed in order to publish each edition. Thank you again for your support and interest in measure. Thaks for telling your readers about "The Measurement Blues." I hope they enjoyed listening to it as much as I enjoyed writing it. I've been kicking around the idea of writing a blues about measurements for five years. I didn't tell anyone about it until I introduced it to the Test & Measurement World staff this year. Of course, I had to get the calibration part in. As we all know, measurements are meaningless without calibration. Martin Rowe Senior Technical Editor Test & Measurement World Richard Pettit Managing Editor Sandia National Laboratories (Retired) H OW TO R EA C H U S: MA I L letters to: NCSLI Te c h n i c a l P a p e r A m e n d m e n t It has been brought to my attention that I inaccurately attributed the development of the first Josephson voltage system (JVS) in my article “Experimental Design of NCSLI 2005 Josephson Voltage Standard Interlaboratory Comparison,” published in NCSLI measure, vol. 1, no. 1, pp. 36-40, March 2006. The first demonstration of a 1 V JVS was performed in the framework of cooperation between Physikalisch-Technische Bundesanstalt (PTB) and NIST. I should have clearly noted this collaboration and should have referenced the following publications: [1] J. Niemeyer, J.H. Hinken, and R.L Kautz, “Microwave-induced constant voltage steps at one Volt from a series array of Josephson junctions,” Appl. Phys. Lett., vol. 45, pp. 478-480, 1984. [2] J. Niemeyer, L. Grimm, W. Meier, J.H. Hinken, and E. Vollmer, “Stable Josephson reference voltages between 0.1 and 1.3 V for high-precision voltage standards,” Appl. Phys. Lett., vol. 47, pp. 1222-1223, 1985. On behalf of myself and my coauthors, I apologize for my omission. Dr. Yi-hua Tang Quantum Electrical Metrology Division National Institute of Standards and Technology (NIST) measure Journal, 2995 Wilderness Pl., Ste 107, Boulder, CO 80301-5404 USA FA X letters to: 303-440-3384 E - M A I L letters to: editor@ncsli.org Vol. 1 No. 4 • December 2006 MEASURE | 3 NMI NEWS NMI NEWS Qualified NMIs Can Now Display the CIPM MRA Logo on Calibration Certificates It is now possible for National Metrology Institutes (NMIs) to display a new logo on their calibration and measurement certificates. This logo, designated as the CIPM MRA logo, consists of the image of the Pavillon de Breteuil designed in blue and surrounded by a yellow arc. This CIPM MRA acronym combines the acronym CIPM, the International Committee for Weights and Measures, and MRA, the acronym commonly used for Mutual Recognition Arrangement. The National Physical Laboratory (NPL) in the United Kingdom was the first NMI to apply to use the new logo. The purpose of the voluntary use of the CIPM MRA logo on calibration certificates is to allow NMIs to draw the attention of their customers, and other interested parties, to the recognition by all other signatories of the CIPM MRA of the validity of those certificates. The logo can only be displayed after a formal application is submitted to the International Bureau of Weights and Measures (BIPM). The CIPM MRA logo can only be displayed on those calibration and measurement certificates covered by a Calibration and Measurement Capability (CMC) document published in the Appendix C section of the BIPM key comparison database (KCDB). For more information, www.bipm.fr/en/cipm-mra/logo/ INMS Moves Forward with Quality System The Institute of National Measurement Standards (INMS), Canada’s NMI, has made considerable progress with their quality system since 2005. Photometry and Radiometry Standards, Acoustical Standards, Electrical Power Measurements (EPM), Dimensional Metrology, and Mass Standards are now listed in Appendix C of the International Committee for Weights and Measures (CIPM) MRA. Appendix C lists the quantities for which calibration and measurement certificates are recognized by NMIs participating in part two of the arrangement. The MRA gives users reliable and quantitative information on the comparability of national metrology services and provides the technical basis for wider agreements negotiated for international trade, commerce and regulatory affairs. The Chemical Metrology, Thermometry, Electrical Standards, Time and Frequency, and Ionizing Radiation Standards laboratories have all recently completed their quality systems and had successful internal audits. The EPM group went through the accreditation assessment visit for their quality system in November 2005. The accreditation body, which was the Standards Council of Canada, assembled an assessment team which included measurement experts from NIST (USA) and NMI (Australia). In response to the assessment findings, all 35 existing procedures were revised and Vol. 1 No. 4 • December 2006 15 new procedures were written. The EPM was then accepted by the Interamerican Metrology System (SIM), the regional metrology organization for the Americas. The Calibration and Measurement Capabilities (CMCs) of the EPM laboratory are now included in the BIPM's Key Comparison Database (KCDB). In 2005-2006 INMS has participated in the planning or implementation of 29 inter-NMI comparisons, including some nine comparisons under the auspices of SIM, which comprises the 34 nations of North, Central and South America, as well as the Caribbean nations. A branch of the National Research Council (NRC), INMS is located in Ottawa and serves as Canada’s NMI. For more information about NRC-INMS: inms-ienm.nrc.cnrc. gc.ca The New Dawn of Time: NPL to Move Time Signal Station The time signal used to set Britain’s clocks with extreme accuracy is on the move from Rugby, where it has been transmitted since 1927, to a new home in Anthorn on the west coast of Cumbria. The signal, often referred to as ‘The time from Rugby,’ will in the future be known as ‘The Time from NPL.’ The National Physical Laboratory (NPL), which has been responsible for the accurate time signal from Rugby since 1950, will make the switch in April 2007 following the award of a new contract to VT Communications. The switchover will take place following a three-month test period at the beginning of next year, with the final transfer from Rugby to Anthorn taking place at the end of March 2007. NPL has reassured most users that they need take no action to continue receiving the signal. NPL managing director, Steve McQuillan, said: “Maintaining accurate time is essential to keeping the modern world working. Most people only need time to be accurate to within a few seconds or even minutes, but global navigation systems, the Continued on page 7 There are three atomic clocks housed at the radio stations. NPL compared these with NPL master clocks in Teddington by making use of the GPS system. The group of atomic clocks at NPL keep the UK's time accurate to within one second in three million years. MEASURE | 5 NMI NEWS internet, email, television, power industry, transportation, and financial systems are just some of the industries that depend on very accurate time to operate. We are delighted to be working with VT Communications on the transfer of the time signal to Anthorn. While most users check their time against the signal periodically, a small number of organizations use the signal constantly in their work. We regularly notify those we know who may be affected by our testing and we’ll be happy to add any other users to our email list if they get in touch. However the vast majority of time signal users will not experience any disruption during the testing and switchover.” Doug Umbers, Managing Director of VT Communications, said: “We are very proud to be working in partnership with NPL on a program of national significance. We are excited to be implementing a highly resilient solution, which will provide tangible benefits to all stakeholders.” The time signal is accurate to within one millisecond of Universal Time and supports a wide range of services, including emergency 999 communications, train companies, cash machines, and mobile phone billing systems. These users will not be affected by the change. The signal’s transmission is linked to NPL’s atomic clocks at Teddington in South West London. NPL is home to UK’s atomic time and one of only five laboratories worldwide using the latest cesium fountain technology to contribute to the world time standard Coordinated Universal Time (UTC). Contact: Joe Bennett, NPL, time@npl.co.uk NIST Noise Measurement May Boost Cell Phone Performance Researchers at the National Institute of Standards and Technology (NIST) and industry collaborators have developed improved methods for accurately measuring very faint thermal “noise” in electronic circuits, which is caused by the random motion of electrons. The technique may help improve the signal range, data rate and battery life of cell phones and other wireless communications devices. Low background noise typically translates to better performance in electronics, such as longer ranges and clearer signals or higher information-carrying capacity. However, noise too low to measure means that circuit designers cannot tune the system for optimal performance. The NIST research focuses on CMOS (complementary metal oxide semiconductor) transistors, which are inexpensive and widely used in integrated circuits for wireless devices. Noise levels for CMOS transistors have, until now, been too low to measure accurately in much of their signal frequency range (1 to 10 GHz), and as a result CMOS circuits may be poorly matched to wireless transmission systems, resulting in significant signal loss. In a collaboration with IBM Semiconductor Research and Development Center (Essex Junction, VT) and RF Micro Devices (Scotts Valley, CA), NIST has developed and demonstrated the capability to reliably measure the noise in CMOS devices before they are cut from silicon wafers and packaged. This is believed to be the first method for on-wafer noise measurements directly linked to national standards for thermal noise Vol. 1 No. 4 • December 2006 power. The new measurement methods were described at the IEEE Radio Frequency Integrated Circuits Symposium in San Francisco.1 The team also demonstrated the use of “reverse” noise measurements, which is focusing on noise emitted from the input of the transistor when incoming signals are reflected and scattered, as a tool for checking overall noise parameters. This method can improve precision, particularly of the optimal impedance properties needed in transistors to minimize noise. Reverse noise measurements also may help improve modeling of CMOS transistors. NIST Researchers Unveil World’s First Quantum AC Voltage Metrology System After 10 years of research, the National Institute of Standards and Technology (NIST) has unveiled the world’s first precision instrument for directly measuring alternating current (AC) voltages. The instrument is being tested for use in NIST’s lowvoltage AC calibration service, where it is expected to increase significantly the measurement precision of industrial voltmeters, spectrum analyzers, amplifiers and filters. Described July 14 at the Conference on Precision Electromagnetic Measurements in Turin, Italy,2 the patented instrument3 is based on the same ‘Josephson junction’ technology used in NIST’s widely used direct current (DC) voltage standards, offering high precision based on quantum physics principles. A Josephson junction consists of two superconducting pieces of metal separated by a thin insulator or normal metal. When a fixed DC voltage is applied across it, a junction responds by generating an AC current that oscillates at a frequency exactly proportional to the applied voltage. The new instrument uses arrays of junctions to generate AC pulses in precisely measured voltage units over a range of audio frequencies. Arbitrary waveforms can be generated at different voltage levels for different applications. The new standard would establish an entirely new method for AC voltage metrology. Until now, AC voltage calibrations have been performed indirectly, by measuring the heat delivered by an instrument to a resistor, and comparing that measurement to the heat delivered by a known DC voltage. At low voltages (such as 2 milliContinued on page 8 1 J. Randa, T. McKay, S.L. Sweeney, D.K. Walker, L. Wagner, D.R. Greenberg, J. Tao, and G.A. Rezvani, “Reverse Noise Measurement and Use in Device Characterization,” Presented June 12, 2006 at the IEEE Radio Frequency Integrated Circuits Symposium, San Francisco, CA. 2 S.P. Benz, C.J. Burroughs, P.D. Dresselhaus, T.E. Lipe and J.R. Kinard, “100 mv AC-DC transfer standard measurements with a pulse-driven AC Josephson voltage standard,” Presented at Conference on Precision Electromagnetic Measurements, July 2006, Turin, Italy. 3 S.P. Benz, C.J. Burroughs, C.A. Hamilton, and T.E. Harvey. U.S. Patent 6,236,344 (issued 5/22/01) “AC And DC Bipolar Voltage Source Using Quantized Pulses.” MEASURE | 7 NMI NEWS volts), the new AC Josephson junction voltage standard should improve measurement accuracy as much as 1,000-fold. The concept for the new device was co-invented by researchers at NIST and Northrop-Grumman in the mid1990s.4 A number of innovations since then have led to the first practical system. For instance, to increase the output voltage, NIST developed “nano-stacked” arrays of Josephson junctions, in which the spacing between junctions is reduced to less than 100 nanometers by stacking the junctions on top of each other. Using this technique, NIST can make programmable voltage standard integrated circuits with over 130,000 junctions on a single chip. The new AC instrument currently has a maximum output of 0.10 volts; NIST researchers hope eventually to increase that level to 1 volt. For more information, contact Sam Benz, NIST: samuel.benz@boulder.nist.gov Einstein’s Magnetic Effect Is Measured on Microscale A gyromagnetic effect, the rotation of an object caused by a change in magnetization discovered by Albert Einstein and Dutch physicist Wander Johannes de Haas, has been measured at micrometer-scale dimensions for the first time at the National Institute of Standards and Technology (NIST). The new method may be useful in the development and optimization of thin film materials for read heads, memories and recording media for magnetic data storage and spintronics, an emerging technology that relies on the spin of electrons instead of their charge as in conventional electronics. The Einstein-de Haas effect was first observed in experiments reported in 1915, in which a large iron cylinder suspended by a glass wire was made to rotate by an alternating magnetic field applied along the cylinder’s central axis. By contrast, the NIST experiments, described in Applied Physics Letters5, measured the Einstein-de Haas effect in a ferromagnetic thin film only 50 nanometers thick deposited on a microcantilever, which is a tiny beam anchored at one end and projecting into the air. An alternating magnetic field induced changes in the magnetic state of the thin film, and the resulting torque bent the cantilever up and down by just a few nanometers. Using a laser interferometer to measure the movements of the cantilever and comparing those data to changes in the magnetic state of the material, researchers were able to determine the “magnetomechanical ratio,” or the extent to which the material twists in response to changes in its magnetic state. The magnetomechanical ratio is related to another important parameter, the “g-factor,” a measure of the internal magnetic rotation of the electrons in a material in a magnetic field. The magnetomechanical ratio and the g-factor are critical in understanding magnetization dynamics and designing magnetic 4`J.X. Przybysz, S.P. Benz, C.A. Hamilton, and A. Worsham. U.S. Patent 5,812,078 (issued 9/22/98) “Josephson Junction Digital to Analog Converter for Accurate AC Waveform Synthesis.” materials for data storage and spintronics applications, but they are extremely difficult to determine accurately because of many potential complicating effects. The NIST experiments In NIST's Einstein-de Haas experiment, the movements of a canprovide a proof-oftilever were measured with an concept for using the optical-fiber laser interferometer. Einstein-de Haas effect The optical fiber is 125 micrometers in diameter, and the end is posito determine the magnetioned less than 10 micrometers tomechanical ratio and from the cantilever surface. the related g-factor in thin ferromagnetic films. The researchers note that a number of improvements are possible, such as operating the cantilever system in a vacuum to reduce the effects of any changes in temperature. For more information, contact John Moreland, NIST: john.moreland@nist.gov NIST Releases New Standard for Semiconductor Industry A wide range of optical electronic devices, from laser disk players to traffic lights, may be improved in the future thanks to a small piece of semiconductor, about the size of a button, coated with aluminum, gallium, and arsenic (AlGaAs). The 1-centimeter square coating, just 3 micrometers thick, is the first standard for the chemical composition of thin-film semiconductor alloys issued by the National Institute of Standards and Technology (NIST). Standard Reference Material (SRM) 2841 was requested by the compound semiconductor industry to help measure and control thin film composition as a basis for optimizing material and device properties. The SRM can be used to calibrate equipment for making or analyzing these materials. Buyers are expected to include companies that grow or characterize thin films or use them to make devices, as well as government and university laboratories. AlGaAs is used as a barrier material to increase conductivity in high-speed circuits for wireless communication; semiconductor lasers for optical disk drives, bar code scanning, xerography, and laser surgery; and light-emitting diodes for remote controls, traffic lights, and medical instruments. The NIST standard is expected to increase the accuracy of chemical characterization of AlGaAs films by an order of magnitude over the current state of the art. Improved accuracy will reduce wasteful duplication of reference wafers, increase the free exchange of thin-film materials between vendors and their customers, and ultimately improve the accuracy of data on relationships between material composition and properties. SRM 2841 can be ordered at http://ts.nist.gov/ts/htdocs/ 230/232/232.htm 5 T.M. Wallis, J. Moreland and P. Kabos. “Einstein-de Haas effect in a NiFe film deposited on a microcantilever,” Applied Physics Letters, September 18, 2006. 8 | MEASURE More NMI News on page 10 www.ncsli.org RUNNING HEAD GOES HERE Vol. 1 No. 4 • December 2006 MEASURE | 9 NMI NEWS NIST Time and Frequency Metrology Group Patents Low-Noise Microwave Oscillator Scientists at the National Institute of Standards and Technology (NIST) in Boulder, CO have patented an oscillator that produces very low noise profiles at microwave frequencies. Oscillators are devices that ideally produce a signal at a very precise frequency which can be used in electronic devices to tell time or to regulate a multitude of functions. As technology improves, there is a continuing need to find precise, costeffective oscillators that work at higher and higher frequencies. Historically, the most precise oscillators have been complicated and expensive to build because of the low-power signals that must be used relative to background noise and the shields and special mounts that eliminate sensitivity to the environment. The best microwave oscillators previously available used solid dielectrics, often coupled with cryogenic systems to reduce background noise, but could not operate without distortion at high power levels. To meet the needs of industry, researchers at NIST took a radically different approach when designing this new microwave oscillator. The resonator uses an open chamber, or cavity, that is about the size of a roll of quarters, rigidly defined by an ultra-stiff ceramic housing. The dimensions are such that any frequency that does not agree with a calDC to 50 GHZ 4/18/06 4:22 PM Page 1 culated, selected resonant frequency of the cavity is rejected. High power in an air or evacuated ceramic cavity makes possible a less complex oscillator with less stringent amplifiers, one of the main sources of oscillator noise. At room temperature, the NIST oscillator produces the same, if not better, results than prior laboratory oscillators. The cavity is small and can be produced using any of several relatively inexpensive, ultra-stiff materials. The result is a simpler, low cost, stable oscillator with reference-standard quality and high resistance to the environment. Extremely stable oscillators have several applications outside of the research environment. A low noise, high-frequency signal is essential for high resolution of radar systems, or for increasing the amount of data sent through communications satellites. In metrology, these oscillators can be used in atomic spectroscopy and as a reference frequency source for microwave tests and calibrations. NIST researchers continue to improve the design, focusing on making it smaller and even more resistant to vibration and severe environmental conditions. For now, the new design represents a significant advance in the development of oscillator technology. Contact: David Howe, NIST, dhowe@boulder.nist.gov Metrology Services www.dynamictechnology.com • 810.225.4601 Chicago 10 | MEASURE Cleveland Dallas/Ft. Worth Detroit www.ncsli.org Vol. 1 No. 4 • December 2006 MEASURE | 11 METROLOGY NEWS METROLOGY NEWS New "Springer Handbook of Materials Measurement Methods" This handbook, published by Springer, presents, for the first time, advanced methods for materials characterization and measurement. Key topics covered in the 22 chapters, include: Measurement Principles and Structures; Measurement Strategy and Quality; Methods for Composition and Structure; Materials Property Methods (Mechanical, Thermal, Electrical, Magnetic, and Optical); Material Performance (Corrosion, Friction & Wear, Environmental Interactions, Condition Monitoring, etc.); International Standards; and Modeling and Simulation Methods. The handbook features over 500 color illustrations, 50 comprehensive tables, and up-to-date approved references. Included with the handbook is a fully searchable CD-ROM for quick data access and links to helpful sources. The 1200 page handbook was published in August 2006. For more information, visit www.Springer.com IAS Requires CCT for Calibration Lab Accreditation by 2009 International Accreditation Service, Inc. (IAS) has stipulated that all IAS-accredited calibration laboratories must have their technicians certified under the American Society for Quality (ASQ) Certified Calibration Technician (CCT) program by December 2009, a move that is expected to raise the bar and provide great benefit to the industry. IAS believes that since the requirement is placed on calibration laboratories, then IAS should lead by example. Hershal Brewer, who leads the IAS accreditation program for calibration, was recently recertified by ASQ under the CCT program. For more information, visit www.iasonline.org CCT Success Story in Tennessee Arnold Engineering Development Center (AEDC), located on the Arnold Air Force Base in Tennessee, now has 15 of their group of 21 specialists earning the American Society of Quality (ASQ) certified calibration technicians (CCT) credentials. That represents over 70% of their calibration personnel. After the first specialists received their CCT certification, they helped teach the most recent preparatory class of six. “All of these individuals either studied and took the course on their own time or went through Motlow State Community College on their personal time,” said David Compton, the manager of the Precision Measurement Equipment Laboratory (PMEL). “They took the initiative to do this, and I salute each of them for reaching this milestone." Jerry Erickson, part AEDC’s PMEL who recently earned his CCT credentials, said: "To qualify for this certifica12 | MEASURE tion and recognition, one must put in some serious study over a period of several months in subjects such as general metrology, measurement systems, calibration systems, applied mathematics, quality systems and uncertainty. The national pass average is about 60 percent, so we are proud to state that the PMEL is at a 100 percent pass rate." All of the center’s newest CCT-qualified individuals are employees of Aerospace Testing Alliance (ATA), the support contractor for AEDC. ATA is a joint venture of Jacobs Sverdrup, Computer Sciences Corp. and General Physics Corp. For more information about AEDC: www.arnold.af.mil LXI Consortium Applauds Web Log Hosted by Test & Measurement World Magazine The LXI Consortium is alerting test-system developers to the availability of the “LXI: Instruments and Applications” Web log (blog) hosted by Test & Measurement World magazine. Moderated by Chief Editor Rick Nelson, the blog can be found at: www.reed-electronics.com/tmworld/blog/1330000133.html The blog’s first entry was posted in April 2006. One key purpose of the blog is to keep readers up to date on the LAN eXtensions for Instrumentation (LXI) standard. LXI is the LAN-based successor to GPIB. The LXI standard goes beyond GPIB to provide additional capabilities that reduce the time it takes to set up, configure and debug test systems. LXI also helps integrators leverage the time and effort already invested in system software and architecture. The standard is managed by the LXI Consortium, a not-for-profit corporation comprised of leading test and measurement companies. The group’s goals are to develop, support and promote the LXI standard. LXI’s flexible packaging, high-speed I/O, and prolific use of LAN address a broad range of commercial, industrial, aerospace and military applications. The site also provides a forum for visitors to ask questions or share their expertise in LXI instrumentation. Recent postings highlight several LXI-related resources, including a series of application notes and an online LXI tutorial created by UKbased www.radio-electronics.com. “We at Test & Measurement World believe it’s important to keep our readers informed about emerging standards and technologies, their benefits and limitations, and the products that conform to them,” said Nelson. “With the emergence of LXI, we thought the blog format would be a great way to post timely information and to encourage readers to share their LXI experiences.” “The LXI Consortium applauds Rick Nelson and the magazine for creating an online forum focused on the LXI standard,” said Bob Stasonis, co-chair of the Consortium’s marketing committee. “We encourage everyone interested in LXI to visit the site and contribute to the online discussions.” Continued on p. 14 www.ncsli.org © 2006 Northrop Grumman Corporation 9mlgeYl]\ [YdaZjYlagf kg^loYj] oal` Y hmdk]& You probably know SureCAL® for its leading calibration software — products that help you maintain your instruments to exacting precision. But we also have a human side, whose hallmark is unparalleled customer service. We believe in the value of an intelligent voice, so we feature live customer support. It’s one more way SureCAL® provides high technology with a human touch. www.northropgrumman.com Search: SureCAL METROLOGY NEWS Additional information about LXI-compliant products, as well as licensing, specifications and consortium membership, is available at: www.lxistandard.org. For more information contact Bob Rennard, President of LXI Consortium, bob_rennard@agilent.com NACLA Announces Operational Changes, Restores Recognition of Two Accreditors The National Cooperation for Laboratory Accreditation (NACLA) has made several recent changes to its operations in order to provide better services to U.S. specifiers, regulators, laboratories and accreditation bodies. With the approval of the current signatories of the NACLA Mutual Recognition Arrangement (MRA), NACLA terminated the MRA effective August 4, 2006. This change was made so that NACLA could offer expanded recognition programs, tailored to meet the needs of government and industry. The NACLA evaluation process will now focus more on customers, but will continue to be based on international and national standards. Following the elimination of the MRA, NACLA made two additional announcements on September 21, 2006. First, it has reinstated two major accreditation bodies (ABs) to the list of NACLA-recognized ABs, and second, it has revised the composition of its Acceptance Panel. 14 | MEASURE The reinstated ABs are IAS (International Accreditation Service) and NVLAP (National Voluntary Laboratory Accreditation Program). Both had voluntarily withdrawn from the NACLA MRA, which necessitated their removal from the NACLA recognition roster. As in the past, NACLA continues to grant recognition solely on the basis of an AB’s demonstrated competence and compliance with both the NACLA requirements and the accepted international standards (ISO/IEC 17011 and 17025). The competence of IAS and NVLAP was amply demonstrated through NACLA evaluations, so their recognition has been restored. With these reinstatements, there are now seven ABs that are recognized by NACLA. “It is important for all NACLA stakeholders to understand that the elimination of the NACLA MRA in no way diminishes the technical rigor or international validity of NACLA’s evaluation process,” according to Dr. William J. Tilstone, NACLA’s President. “The standard of competence that has been used in NACLA’s evaluation program from the start will still be applied. The main difference from our previous MRA-driven system is the lack of a requirement that the AB, once its competence has been ascertained, will no longer be asked to sign a NACLA MRA.” The second change occasioned by the elimination of the NACLA MRA is a restructuring of the Acceptance Panel. This was done to increase the proportion of government and industry representatives on the Panel, thus giving users of accredited laboratory results more input into the recognition process. “At www.ncsli.org METROLOGY NEWS the end of the day, the clients for laboratory data are government agencies and private-sector companies,” Dr. Tilstone said. “It is appropriate, therefore, that representatives of these organizations should have the primary role in deciding which ABs merit recognition.” For more information: www.nacla.net Agilent Labs’ Len Cutler Leaves a Lasting Legacy Agilent Technologies mourns the loss of Len Cutler, Agilent Distinguished Fellow and member of the technical staff in Precision Instrumentation at Agilent Laboratories. After a career of 57 years, Len passed away September 4, 2006 while camping with his family in Big Basin, California. He was an internationally recognized inventor who made significant contributions to the worlds of science and technology, particularly in precision time measurement and laser interferometry. In 2004, Agilent Laboratories promoted Len to Agilent Distinguished Fellow in recognition of his long-standing and farreaching contributions that have had, and will have, an enduring impact on the company. Len was the first and only person to hold this position, Agilent’s highest technical honor. “We honor Len for leaving a lasting legacy, and acknowledge his leadership as an innovator and researcher at HewlettPackard and Agilent for almost 50 years,” said Darlene Solomon, chief technology officer and vice president Agilent Laboratories. “Len set the standard for world class research; he served as a mentor to so many of our engineers and scientists at Agilent Labs, and remarkably, continued as an active contributor to our research program until his passing. We have lost a great man, a brilliant researcher, a wise leader and a good friend.” Len has been aptly named “Father Time.” Over the past 40 years, his innovations and inventions have led to the world’s most accurate commercial time keeping devices, beginning with the first solid-state atomic clock in 1964, and leading up to the Hewlett-Packard 5071A cesium clock, introduced in 1992, with an accuracy of one second in every 1.6 million years. Clocks designed by Len form the cornerstone of the time standard maintained by laboratories throughout the world. His clocks were the first to be flown in airplanes to perform the synchronization of world clocks and later to establish the variations in the flow of time predicted by Albert Einstein. The impact of this work is crucial to modern commerce. Without accurate time keeping, there would be no GPS navigation, modern computer networks would no longer function, and financial transactions would grind to a halt. Lyons and Sherwood built the first atomic clock in the 1950’s at the National Bureau of Standards based on theoretical work by Maxwell and Rabi. Pioneering work by Townes, Zacharias, Essen, and Ramsey then led to the first cesium beam clock. Len Cutler began his work on atomic clocks at Hewlett-Packard in 1959, introducing the first solid state cesium beam clock in Continued on page 17 Vol. 1 No. 4 • December 2006 MEASURE | 15 1964. The performance and reliability of these clocks were much better than anything previously available. Over the years, Len and his team made many improvements including fundamentally new techniques, like trapped ion frequency standards, and optically pumped atomic clocks, as well as many contributions to the optimization and theoretical understanding of the cesium clock. He has authored a total of 25 patents in many areas of science and technology. Perhaps his most important invention is his method for the precise measurement of distance using a two-frequency laser interferometer system. This invention is the crucial element in the step-and-repeat lithography systems used for the manufacturing of silicon integrated circuits, in which nanometer resolution is required. Len’s reputation for innovation resulted in his consultation on the high-visibility failure of one of the nation’s premier high-technology rapid transit systems in the early 1970’s. After a serious accident on a major U.S. transit system, he and his colleagues were quickly able to invent a patented logic safety system to prevent future incidents. In addition to his scientific and technical contributions, Len was one of the founders of Hewlett-Packard Laboratories and was the leader and mentor of several generations of researchers in that lab and its successor, Agilent Laboratories. NCSLI, NIST, and CPEM Sign Agreement Reprinted from: www.agilent.com/labs/news/2006features/ fea_cutler.html For more information: www.icpem.org/ Vol. 1 No. 4 • December 2006 The Conference on Precision Electromagnetic Measurements (CPEM), NIST and NCSLI have signed a sponsorship agreement under which CPEM2008 will be cohosted by the NCSLI and NIST Boulder Laboratories. CPEM2008 will be held in Broomfield, CO, near Boulder, June 8–13, 2008. The proximity of NIST Boulder and the NCSLI Business Office allows the burden of running CPEM2008 to be easily shared between NIST Boulder and NCSLI. Basically, NIST will be responsible for all aspects of the conference excepting the finances, which will be handled by NCSLI. This agreement builds on the long historical relationship between CPEM and NCSLI, both having been founded in Boulder in the 1950s as a result of metrologists seeing the need for better communications throughout government and industry. The CPEM holds a conference every two years as a means of disseminating information concerning precision electrical measurement principles and techniques relevant to standards and measurements, the science underlying them, and their application to practical measurement problems. CPEM2006 was held this summer in Torino, Italy, and CPEM2010 will be held in Korea. MEASURE | 17 We’re in your neighborhood Let us pick up and deliver — or calibrate your equipment at your site With six regional labs, it’s easy to access Agilent’s calibration services. And when the original manufacturer calibrates your instruments, you can be sure that they retain their “like new” performance Remove all doubt and accuracy. Your instruments restored to like-new condition, returned on time Thorough, high quality calibration starts with automated, factory- • Premier measurement expertise • NIST-traceable metrology • A2LA-accredited calibration available written procedures. All necessary adjustments are included, no matter how many test points are out of cal—and you’ll receive comprehensive pre- and post-adjustment data. We’ll even advise you of the latest hardware and firmware updates, clean and lubricate parts that need it, and make minor repairs—all at no extra cost. For more information, send email to service-info@agilent.com or call 1-800-829-4444 and we’ll connect you to your local cal center: Arlington Heights, IL Bethlehem, PA Chelmsford, MA Durham, NC Irvine, CA Richardson, TX www.agilent.com/find/removealldoubt © Agilent Technologies, Inc. 2006 Instrument availability is more predictable too. Turnaround time from your local cal center is normally five business days, and if you’re within a two-hour radius of your local cal center, pick up and delivery are free. Or when downtime is critical, our Volume Onsite Calibration (VOSCAL) service brings Agilent’s expertise, equipment and personnel to your site. To learn more, drop us a line at service-info@agilent.com. We’ll reply with an overview of your nearest cal center’s capabilities and a copy of our “Remove all doubt” brochure. RUNNING HEAD GOES HERE 2007 NCSL International Workshop & Symposium Metrology’s Products Impact Services JULY 29 – AUGUST 2 Saint Paul RiverCentre Saint Paul, Minnesota on and Every product and service that consumers use is highly dependent on metrology. From the fit and finish of our vehicles to weights and volumes of products purchased in the grocery, we are impacted at every level. www.ncsli.org/conference CallForPapers@ncsli.org • Exhibits@ncsli.org 303-440-3339 info@ncsli.org Metrology laboratories calibrate equipment used to create compatible component parts used in commercial and consumer products. A sound and cohesive metrology and quality system, from the National Metrology Institute to the end consumer, impacts the quality of life for everyone. RUNNING HEAD GOES HERE 20 | MEASURE www.ncsli.org QUAMETEC INSTITUTE OF MEASUREMENT TECHNOLOGY METROLOGY & ISO17025 CONSULTING SERVICES COURSES IN: Metrology & ISO17025 • Public Classes • Self-Paced CD/DVDs • Onsite Training • Coming Soon: eLearning Courses • • • • • • Quality Systems Gap Analysis Compliance Audit Uncertainty Analysis Metrology Consulting Instrumentation QUAMETEC PROFICIENCY TESTING SERVICES MEASUREMENT QUALITY VALIDATION • • • • • Small Uncertainties Quick Test Results PT Planning Custom Test Design A2LA Accredited CALL FOR MORE INFORMATION CALL FOR MORE INFORMATION CALL FOR MORE INFORMATION 810.225.8588 810.225.8588 260.244.7450 Vol. 1 No. 4 • December 2006 MEASURE | 21 SPECIAL REPORT The CIPM Working Group on Metrology of Materials S eton B en net t and Grah am Si ms A b s t r a c t : Following international discussion of the traceability issues arising in the measurement of materials prop- erties, the Comité International des Poids et Mesures (CIPM) in October 2005 accepted the proposal to set up an ad hoc Working Group on Metrology of Materials (WGMM). The WGMM is assessing a wide range of materials properties, looking particularly at the need for improved traceability routes, data comparability, and the availability of appropriate reference materials. The Working Group on Materials Metrology will report to CIPM in October 2007, with the intention of raising the profile of materials metrology internationally and engaging the leading National Measurement Institutes in recognising and addressing known difficulties in demonstrating traceability of many material properties to the Système International of units (SI). Terms of Reference have been agreed, and the first meeting took place at the United Kingdom’s National Physical Laboratory in May 2006. This paper describes the range of properties being investigated and highlights the studies being undertaken by the WGMM members. 1. M e a s u re m e n t s , Tr a c e a b i l i t y a n d S t a n d a rd s In most areas of metrology, the concept of traceability to established national or international standards is well understood. The SI (Système International) provides a coherent set of well-defined units which provide a common language for expressing and understanding the results of measurements. The national metrology institutes (NMIs) maintain Seton Bennett and Graham Sims National Physical Laboratory Teddington TW11 0LW United Kingdom Email: seton.bennett@npl.co.uk 22 | MEASURE standards according to their national needs and the equivalence of these standards has been established through the mechanisms of the Comité International des Poids et Mesures (CIPM) Mutual Recognition Arrangement which includes: Key intercomparisons, mutual review of claimed capabilities, and regional assessment of NMI quality systems. The result of this methodology is that NMIs can generally demonstrate excellent agreement in intercomparison exercises in these classical fields of measurement, and they pass this metrological confidence on to accredited testing and calibration laboratories which are in turn required to demonstrate the traceability of their results. Figure 1 shows the results of measure- ments of the length of a 175-millimetre gauge block by eleven NMIs. [1] All the results agree within 1 part in 106 and, with one exception, the agreement is better than 3 parts in 107. This impressive result reflects a very thorough and careful approach to the relatively straightforward measurement of a length standard. Similar exercises to evaluate the ability of laboratories to measure materials properties are often less impressive. Figure 2 illustrates the results of a national intercomparison exercise in the UK to determine Young’s modulus. [2] The results from 25 laboratories for a simple modulus measurement show a spread of nearly 25 %. There are many possible reasons for this enormous diswww.ncsli.org SPECIAL REPORT Vol. 1 No. 4 • December 2006 150 100 50 0 –50 -100 –150 –200 CNR-IMGCT PTB NPL NIST INMETRO NRC NMIJ NIM CSIRO-NIML CSIR-NIML VNIIM F i g u re 1. Results of Consultative Committee for Length (CCL) Key Comparison of 175-mm gauge block. [1] 25 12.5 0 –12.5 N04 N13 N07 N16 N05 N06 N01 N22 N08 N10 N11 N24 N02 N18 N12 N17 N09 N19 N03 N20 N15 N25 N14 –25 N26 The need for wider international collaboration in the measurement of materials properties, and in particular the issues of standards and traceability, have been under discussion for a number of years at meetings of the Versailles Project on Advanced Materials and Standards (VAMAS). [3] VAMAS operates under a Memorandum of Understanding signed by senior representatives of government in countries of the Economic Summit (G7) and of the European Community. It supports international trade through projects aimed at providing the technical basis for drafting codes of practice and specifications for advanced materials. The scope of this international collaboration embraces all aspects of enabling science and technology required as a precursor to the drafting of standards. Through its activity, VAMAS fosters the development and harmonisation of international standards for advanced materials by the various existing standards agencies. Since its inauguration in 1982, VAMAS has had a considerable impact on the development of internationally 1 7 5 mm S te e l Ga u ge Bloc k , S /N 6 0 7 1 200 N23 2. I n t e r n a t i o n a l C o n c e r n a nd th e R ol e o f the C IPM CC L - K 2 D e g re e s o f e q u i v a l e n c e [ D j 1 a n d i t s e x p a n d e d u n c e r t a i n t y ( c o v e r a g e f a c t o r : 2 ) U j i ] Va r i a t i o n i n You ng ' s M o d ul us , % crepancy between laboratories, but the contrast with the level of agreement obtained by NMIs comparing results for the calibration of a length standard could hardly be more marked. A key question is to differentiate between intrinsic properties of materials and other parameters related to the form and scale of a material specimen. Thus thermal expansion or Young’s modulus are quite clearly intrinsic properties, while surface finish or particle size describe individual samples and may effect the results of measurements of properties. The use of standardised measurement procedures, as for hardness, creates a repeatability which depends on careful adherence to the accepted measurement sequence and a form of traceability when every laboratory uses the same procedure. Separating the properties of materials from the factors and problems which influence the results of measurements is a clear prerequisite for any study of genuine material property measurement traceability issues. L ab o ra t o ry Co d e N u mbe r F i g u re 2. Results of round robin determination of Young’s modulus. [2] accepted standards for engineering materials. The specification of materials in terms of their characterisation and their performance is based mainly on measurement methods and procedures, with a lack of emphasis on the need for reliable traceability. In 2004, the VAMAS Steering Committee approved actions aimed at bringing about closer collaboration between VAMAS and CIPM. Seeing the need to widen participation in their activities, Dr. Graham Sims (VAMAS Chairman) wrote to Professor Andrew Wallard, BIPM Director, drawing his attention to the need to include materials in the formal international structure for metrology by engaging the attention of NMIs and the CIPM. They saw this as the way to bring international authority and metrological experience to the hugely important and growing area of materials metrology. Following discussion at the meeting of NMI Directors in September 2004, MEASURE | 23 SPECIAL REPORT BIPM hosted a workshop in February 2005 to explore the issues, identify specific traceability questions in materials science and propose further international initiatives in the field. The conclusions of this workshop informed a discussion at the 2005 meeting of the CIPM, which decided to set up an ad hoc Working Group on Materials Metrology (WGMM). P ro p e rt y A re a Detailed Aspects Mechanical Properties Hardness Modulus Strength 3. T he C I P M Wo r k i n g G ro u p o n M a t e r i a l s M e t ro l o g y Following the 2005 CIPM decision, the ad hoc Working Group on Materials Metrology has been established with experts from NMIs and other institutes in some 10 countries. Its Terms of Reference are to: 1. Identify those material properties for which globally comparable, traceable measurement results are important for science, engineering and manufacturing technology; 2. Identify those material properties for which the needs for traceable measurements are not covered by the activities of the Consultative Committees; 3. Establish the user needs for activity in materials metrology; 4. Investigate the existing capabilities of participating NMIs by initiating some pilot studies, including a small number of interlaboratory comparisons; 5. Develop tools and methodologies for establishing traceability in materials testing; 6. Define the objectives, aims and initial activities for an ongoing programme in metrology for materials, including recommendations for underpinning activities, such as the organisation of Key Comparisons and the development of Reference Materials and Reference Methods; 7. Liaise closely with other interested organisations; and 8. Report its conclusions to the CIPM by October 2007. Toughness Fatigue Creep Viscosity Thermophysical (Phys-Chem) Properties Conductivity Diffusivity Expansion Specific Heat Emissivity Composition and MicroStructural Properties Grain size / boundaries Phase Porosity Texture Particle size Defects Functional Properties Electrical Optical Magnetic Thermo-electric 4 . In iti al S tep s The first meeting of the Group, in May 2006, began with a discussion of the materials properties which are important for science and manufacturing, and explored some of the issues associated with establishing traceability to appropriate standards. To the list of obvious properties (mechanical, electrical and thermal coefficients of solids) were quickly added a number of properties of liquids and the distinct properties of materials on the nano-scale (see Table 1). No experimental studies or intercomparisons are planned at this stage, as the Working Group on Materials Metrology decided it was more important to concentrate on collecting information about previous exercises and to study existing provisions for traceability when materials properties are being measured in testing laboratories and elsewhere. The Group recognised that in some cases traceability may be to a standard or a procedure, rather than to the SI in the generally accepted sense, and that the reliability, repeatability and reproducibility of results will be affected by a number of factors. There is also a need to seek the views of the user community in order to identify those properties for which repeatability and comparability are particular problems. Table 2 identifies some of the user aspects and needs for comparability of data that are used in support of regulations and certification. 24 | MEASURE Acoustic Electrochemical Properties Ta bl e 1. Material properties to be initially assessed. 5. The Wa y A h e a d The Working Group on Materials Metrology will meet again in December 2006. Meanwhile, as well as collecting historical data about previous intercomparisons and identifying issues associated with specification standards and measurement procedures, an attempt will be made to obtain views and information from the user community, including answers to the following questions: • For which material properties is it particularly difficult to demonstrate traceability as the “property of the result of a measurement . . . whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties?” (definition from the International Vocabulary of Metrology (VIM), 1993) • For which measurement results is it difficult or impossible to express in terms of SI units? • Where is there a particular need for intercomparisons to www.ncsli.org SPECIAL REPORT A sp e c t For each property (e.g., Modulus) Scope Short description of property and measurement methods/techniques. Material Category Meta, ceramic, polymer, composite, rubber, etc. Material State and Scale Liquid, micro, particle, bulk, film, nano, surface Regulation Need Is this property a directly or implied requirement in any regulative document (e.g., law; EU directive; industry (CAA, DNV, Lloyds) regulation; etc.)? User Needs What are the user requirements (excluding the regulatory aspects listed above)? Accreditation Needs Is this property needed in any accreditation procedure? Comparability Is there a need for intercomparability of data (e.g., if required in design codes, when more than one code and/or test method exist for the same application, such as pressure vessels)? Need for Intercomparison Should the WGMM initiate intercomparisons or recommend future exercises? Economic Impact Studies Have any economic studies been undertaken of the impact of incorrect or high scatter measurements of this property? Number of Existing Methods How many “standardized” or accepted methods are used to measure this property? Standardisation Situation What standards exist and do they include precision data? NMI or Metrology Research Institute Activities Is there any activity in this property measurement at these organizations? Research & Development Phase? Is there any basic research into this property and/or new measurement techniques? Traceability Issues What is the traceability route for this property? SI units Relevant Which SI unit (if any) is relevant? CCs Coverage Is this property covered by any existing BIPM Consultative Committee (CC)? Prior Studies (Method Specific) Have any prior studies been undertaken for this property? Method Comparability Have any prior comparability studies been undertaken for this property? Standard Test Machines Do standard test machines exist for these measurements? establish equivalence and repeatability between different laboratories and/or various procedures? Following the December 2006 meeting, the Working Group on Materials Metrology will undertake further work, possibly including some very limited pilot intercomparisons, before preparing its report for the CIPM meeting in November 2007. This report may include recommendations for new initiatives to improve the comparability and traceability of the measurement of materials properties worldwide. 6. Conclus ion This paper is based on a presentation to the NCSLI Workshop & Symposium in Nashville in 2006. Our wish is to put these questions to a wider audience, and we invite readers in testing laboratories, accreditation bodies, materials producers or manufacturers, who have a need to obtain accurate values for the properties of the materials they use, to contact the Working Group on Materials Metrology. Information about specific requirements and case studies on past difficulties are of particular value, especially when supported by references or detailed measurement data. You can contact us by e-mail at seton.bennett@npl.co.uk or graham.sims@npl.co.uk. 7 . Re f e re n c e s [1] A. Lewis, CCL-K2, “Long Gauge Block Measurement by Interferometry: Final Report,” Metrologia, vol. 40, Tech. Suppl., no. 04004, 2003. [2] B. Roebuck, J.D. Lord, P.M. Cooper and L.N. McCartney, “Data Acquisition and Analysis of Tensile Properties for Metal Matrix Composites,” ASTM J. Testing and Evaluation, JTEVA, vol. 22(1), pp. 63-69, 1994; and presented at the ASTM Workshop on Accuracy of Load and Strain Measurements, Miami, FL, Nov. 18, 1992. [3] For more information on VAMAS, see the web site www.vamas.org. Have machines been compared? Calibration – Reference Materials Do calibration reference materials exist for this property? Tabl e 2. Aspects of material property use, traceability and support to regulations. Vol. 1 No. 4 • December 2006 MEASURE | 25 SPECIAL REPORT ANSI/NCSL Z540.3:2006: Requirements for the Calibration of Measuring and Test Equipment D el Caldwell A b s t r a c t : This paper provides an introduction to the new ANSI/NCSL Z540.3 standard and its approach to prescrib- ing requirements for a calibration system that controls the accuracy of the measuring and test equipment used to ensure that products and services comply with prescribed requirements. The new ANSI/NCSLI Z540.3 standard replaces Part II of the current standard, ANSI/NCSL Z540-1 (R2002). Z540.3 consists of six clauses, each of which is described: Scope; References; Terms and Definitions; General Requirements; Calibration System Implementation; and Calibration System Assessment and Improvement. This paper also discusses the three requirements that received the most interest and discussion during the development process: (1) Measurement decision risk criteria; (2) Test uncertainty ratios; and (3) Use of calibration laboratories accredited to ANS/ISO/IEC 17025. 1. I nt ro d u c t i o n On 3 August 2006, NCSL International received a notice from the American National Standards Institute (ANSI) that ANSI/NCSL Z540.3-2006, Requirements for the Calibration of Measuring and Test Equipment had been approved by ANSI’s Del Caldwell Caldwell Consulting Group 906 Pomona Ct. Claremont, CA 91711-3864 USA Email: del.caldwell@verizon.com 26 | MEASURE Board of Standards Review as an American National Standard. This announcement culminates the efforts of Working Group One of NCSLI’s 174 Standards Writing Committee that began in early 2003. ANSI/NCSL Z540.3 “prescribes requirements for a calibration system to control the accuracy of the measuring and test equipment used to ensure that products and services comply with prescribed requirements.” The new American National Standard is intended to replace Part II of the current standard, ANSI/NCSL Z540-11994(R2002) [1], which addressed compliance requirements for a calibration system for measuring and test equipment. With the recent adoption by ANSI of ISO/IEC 17025, “General www.ncsli.org SPECIAL REPORT Requirements for the Competence of Testing and Calibration Laboratories” as an American National Standard [2], the basic functionality of Part I of the current standard, is also replaced. Part I of the current standard includes a mix of compliance and competency demonstration requirements for a calibration or standards laboratory. As a result of these two actions, the stage has been set to rescind the current Z540-1 standard in mid-2007. 2 . Pu r p o se o f Z5 4 0 . 3 In many organizations, measuring and test equipment are used in the research, development, test, evaluation, production, and support of products and services to a wide range of customers. The information gained from the use of the measuring and test equipment contributes to our knowledge of the organization’s product or service and to the associated decisions about their quality and suitability for their intended application. The validity of measurement results are significantly affected by the uncertainty of the measuring and test equipment, which is of particular importance to the success of the organization. This new American National Standard describes a management system used to ensure the continued accuracy of measuring and test equipment used to support the organization’s endeavors. This approach is described in the form of a calibration system or program. Measuring and test equipment that affect the quality of the organization’s product or service and their conformity to requirements need to be included in the calibration system. Performance requirements for the measuring and test equipment are determined and subsequently used as a basis or guiding criteria for the system’s calibration support of the equipment. The measuring and test equipment are periodically calibrated using a defined process that ensures that the calibration results are traceable to the International System of Units (SI), usually through the U.S. National Institute of Standards and Technology. Monitoring and controlling the performance of the equipment are part of the calibration system’s functions. As a result, personnel using the calibrated measuring and test equipment should be able to place confidence in the performance of the equipment included in the system. The new Z540.3 is a requirements oriented document and suitable for use in contract applications. Therefore “how to” type information is limited to guidance in notes, as implementation of the new Standard may vary within and among organizations. However, additional descriptive information is planned to be included in an NCSLI Handbook that is currently under preparation by the NCSLI 171 Calibration System Resources Committee. 3 . C on t en t s o f Z5 4 0 . 3 ANSI/NCSL Z540.3 is comprised of six major clauses: Clause 1: Scope Clause 2: References Clause 3: Terms and definitions Clause 4: General requirements Clause 5: Calibration system implementation Clause 6: Calibration system assessment and improvement The basic content of each clause is described in the following paragraphs. Figure 1 is provided to illustrate the implementaVol. 1 No. 4 • December 2006 tion and effect of Clauses 4, 5, and 6. The scope is included in Clause 1 and has been described in the previous introductory statements above. There are two types of references included in Clause 2 of the new Standard: Normative and Informative. One normative reference is included, the VIM, to provide a reference for terms not defined in the Standard. Three informative references are included that apply only to the extent described. These include: ANSI/NCSL Z540-2-1997 (R2002) (the equivalent of the ISO GUM) [3]; ANS/ISO/IEC 17025:2005 [2]; and NCSLI RP-11996 [4]. These documents provide supplemental information on the expression of measurement uncertainty, calibration laboratory competency assessment, and calibration interval establishment and adjustment, respectively. There are thirteen terms and definitions that have been included in Clause 3. These terms and associated definitions were included where either the definition in the VIM needed to be adapted to reflect common U.S. usage or the term was not defined in the VIM. Some of the terms are common but can have multiple meanings in everyday usage. Accordingly, they were included to be specific about their application in the new Standard. Clause 4 addresses the general requirements that apply throughout the calibration system and an organization’s implementation of that system. It includes the fundamental objective: “establish, document, operate, and improve a system to manage the calibration of measuring and test equipment. The organization shall identify and include measuring and test equipment in the calibration system having an influence on the quality of the organization’s product and its conformity to determined requirements.” Of course, the clause also includes the requirements for identifying the performance requirements of the measuring and test equipment based on their application and using this information as the principal drivers for the technical aspects of the calibration system, such as, measurement reliability, calibration service quality, calibration tolerances, etc. This clause also includes requirements for quality objectives, personnel, information resources, and shipping and handling of measuring and test equipment. Clause 5 describes the technical requirements for implementing and sustaining a calibration system. This clause includes the following components: 5.1 Calibration requirements. This sub-clause basically reiterates the requirement for organizations that utilize measuring and test equipment to provide a product or service are required to identify those equipments that have an influence on the quality of the organization’s product or service and include them in the calibration system. 5.2 Measuring and test equipment. The focus of this subclause is the requirements for identification of measuring and test equipment that are included in the system, including their calibration status. Management of adjustment means and nonconforming measuring and test equipment are also addressed. 5.3 Calibration of measuring and test equipment. The principal technical requirements for calibration of measuring and test equipment are addressed in this sub-clause. Included are requirements for: MEASURE | 27 SPECIAL REPORT Calibration System Product-Service: • Research Calibration System: General Requirements • Calibration System Implementation • Calibration System Assessment and Improvement • Development • NIST & SI Calibration Quality Requirements • Production • Test Calibrated M&TE & Inspection Customer • Support • Operations • Maintenance Measurement Reliability & Calibration Intervals M&TE Application Requirements Traceability Suppliers M&TE Calibration Requirements Assessment & Improvement Calibration Service Requirements F i g u re 1. Calibration system of an organization controlling measuring and test equipment (M&TE). • • • • • • • Measurement uncertainty for reporting measured values; False accept risk or test uncertainty ratio for tolerance tests; Calibration procedure objectives, content, and validation; Measurement assurance procedure objectives and content; Measurement uncertainty and traceability; Calibration equipment availability and application; Calibration personnel competency, supervision, and authorization; • Influence factors, condition monitoring, and control; • Calibration quality monitoring; • Calibration reporting and reports; and • Calibration records. 5.4 Calibration intervals. This sub-clause addresses requirements for establishing and adjusting calibration intervals of measuring and test equipment that are included in the system in order to assure acceptable measurement uncertainty, traceability, and reliability requirements are achieved. The potential for conflicts of interest is addressed, along with the requirement for recall of measuring and test equipment at the end of their calibration interval. Emphasis is also placed on measuring process control intervals. 5.5 Outside suppliers. Z540.3 addresses requirements for outside suppliers if their product or service have an effect on the calibration system. Further, if the outside supplier is acting as a 8 component of or supplements the calibration system, the supplier is required to meet the applicable requirements of the new Standard. For example, if the organization contracts with an outside supplier to perform measurement uncertainty analysis 28 | MEASURE for calibrations being performed within the system, then that supplier must meet the applicable parts of the new Standard. Clause 6 addresses the requirements for assessment of all aspects of the calibration system to ensure conformity of the system with the new Standard, determine the suitability and effectiveness of the system, and improve the system as needed. The overall assessment and improvement activities are organized into five areas: 6.1 Management review. A periodic review to ensure the adequacy, effectiveness, and suitability of the system to achieve established objectives. 6.2 Calibration system audit. An independent audit of the calibration system to established criteria. 6.3 Calibration system monitoring. Monitoring of critical system parameters including: • Performance quality of measuring and test equipment; • Calibration interval suitability; and • Calibration service quality and traceability. 6.4 Customer assessment, verification, and feedback. Addresses involvement by the customer in assessment and verification, including use of customer feedback information. 6.5 Corrective and preventive action. Includes the requirements to identify causes of unacceptable performance and to eliminate the discrepancies. 4. I m p ro v e m e n ts I n c l ud e d i n Z 5 4 0 .3 During the development of Z540.3, members of Working Group One took a broad look at what the new Standard offered www.ncsli.org SPECIAL REPORT over continued use of Part II of ANSI/NCSL Z540-1. In general, the new standard provides mechanisms to improve the quality of measuring and test equipment performance, its management and the related calibration services by: • Incorporating experience with the use of previous and associated standards to improve the criteria for a calibration system and its components; • Adding the requirement to identify measuring and test equipment application related performance criteria to assure the compatibility of calibration intervals and calibration services to application requirements; • Improving requirements for calibration procedures including: addressing the objectives of their use, providing tolerance test criteria, providing measurement risk management criteria and guidance, clarifying test uncertainty ratio determination, increasing topics to be addressed, and providing validation criteria for their suitability; • Requiring traceability to the SI units and improving compatibility with the U.S. National Measurement System; • Adding requirements for procedures and documentation of measurement uncertainty determination and expression to improve the application to calibration; • Providing improved criteria and supplemental guidance for calibration intervals and the use of measurement assurance processes for management of measuring and test equipment performance; • Extending requirements to control operator accessible calibration adjustments; • Improving requirements for identifying non-conforming measuring and test equipment and addressing the impact of their use; • Providing requirements for optional use of accredited calibration services; and • Adding requirements for assessment, quality control monitoring, and system improvement. Of these, three topics raised the most interest and discussion: measurement risk criteria; test uncertainty ratios; and use of calibration laboratories accredited to ANS/ISO/IEC 17025. Sub-clause 5.3 addresses requirements for two types of calibrations: one where measured values are reported to the customer; the other for calibrations involving tolerance tests. For the first type of calibration, the measured values are reported with a measurement uncertainty that is required to be acceptable to the customer. The second states that “the probability that incorrect acceptance decisions (false accept) will result from calibration tests shall not exceed 2% . . .” This is commonly called false accept risk. An option to this requirement is to ensure that the test uncertainty ratio is equal to or greater than 4:1. Here, test uncertainty ratio is simply the ratio of the range or span of the tolerance to twice the 95% expanded uncertainty of the measurement process used for calibration (this applies to two-sided tolerances). Note that both approaches require determination of the measurement uncertainty. These two approaches can provide comparable calibration quality for workload with mid- to low-measurement reliability. The reason for using both the false accept risk and the test uncertainty ratio approaches is that if you have reasonable information about the Vol. 1 No. 4 • December 2006 workload to determine false accept risk, you have greater options for achieving the requirements, such as: adjustment of calibration intervals for different measurement reliability levels; use of guard bands for management of test tolerances, etc. However, if you do not have reasonable information about the workload, then you could still make use of the test uncertainty ratio requirement. It should be noted that by using the false accept risk approach, there are many scenarios where you can achieve an acceptable level of risk and yet the test uncertainty ratio is less than 4:1. Both sub-clauses 5.3 and 5.3.3.2 require that all calibration servicing components, such as a calibration or standards laboratory or facilities associated with the calibration of measuring and test equipment that are included in a calibration system, be competent. Competency includes either meeting the full requirements of Z540.3 or being suitably accredited to, or found to be in conformance with, the requirements of ANS/ISO/IEC 17025, including meeting the requirements of the overall sub-clause 5.3. Sub-clause 5.3 acknowledges that accreditation to ANS/ISO/IEC 17025 may be an acceptable means to recognize a calibration service’s competence provided that the scope of the accreditation is compatible with the specific measuring and test equipment calibration requirements and the additional requirements of this sub-clause are included in the accreditation process. This should provide reasonable assurance that the goals of Z540.3 are met. Sub-clause 5.3 also allows for the customer to accept alternative, independent assessments of conformance, such as audits, evaluations, and inspections, which clearly validate the calibration service’s capability, competence, and compliance with Z540.3 to the customer’s calibration requirements. In either case, all such assessments are to be made for specific calibration capabilities and competence which match the parameters included in the customer’s measuring and test equipment calibration requirements and which are listed in documentation which attests to their authenticity. 5. Sum m ar y a nd C onc lus i on During the development of ANSI/NCSL Z540.3, Working Group One initially invested a significant amount of time and effort to explore modeling the new Standard in the same form as the current standard, ANSI/NCSL Z540-1. We included in the model two international standards, ISO/IEC 17025 and ISO 10012, combining extended versions in a single document to play the roles of Part I and Part II of Z540-1, respectively. We found that the resulting document was excessively long and, due to overlaps and other harmonization issues, difficult to comprehend and use, even though the document in itself was technically acceptable. Accordingly, the final approach described here was determined to be the best approach for expressing the requirements for the calibration of measuring and test equipment, as well as fulfilling needs that are unmet by other standards and doing it to take advantage of one of the international standards (ANS/ISO/IEC 17025). Working Group One was also conscious of the need to express the requirements in a straightforward and explicit MEASURE | 29 SPECIAL REPORT manner to facilitate their implementation and interpretation. As with any change, there are costs and benefits with the use of the new Standard. We believe that the benefits outweigh the modest costs. One colleague who is associated with a major aerospace firm adapted their calibration system to the requirements of Z540.3, even before it was finished. His view was that the changeover went smoothly and was relatively easy as their practices, for the most part, addressed the requirements. Z540.3 has been formatted for publication and will be included in the NCSLI publications CD provided to all NCSLI member organizations. Z540.3 will also be available for purchase through the NCSLI Business Office, Boulder, CO. 6. Ac kn o wl e dg em e n t As the Working Group One chairperson, I wish to thank all the members for the special contributions that each made throughout the development of Z540.3. The explorations and discussions were open and candid, with each member working to fulfill the goal of serving the U.S. measurement community. Members and alternate members of Working Group One, together with their affiliation, included: Dave Abell, Agilent; Del Caldwell, CCG; James Clark, Boeing Commercial; Marcel Dubois, Anritsu; Bill Eyler, Agilent; Doug Faison, NIST/ NVLAP; Chet Franklin, DynCorp; Bob Fritzsche, NSWC Corona; John Grajera, Lockheed Martin; Dan Harper, Harper Quality; Jerry Hayes, Hayes Technology; Del Knapp, Tektronix; Ray Kotowski, NASA; Mark Kramer, USMC; Brian Lee, 30 | MEASURE Anritsu; Bill McCullough, CSC; Paul Nelson, Raytheon; and Derek Porter, Boeing Commercial. We are also indebted to many others in the community for their contributions, including: Alane Caldwell, IRES; Howard Castrup, ISG; Dave Deaver, Fluke; Dennis Jackson, NWSC Corona; and Larry Nielson, SCE. 7. R e f e re n c e s [1] “Calibration Laboratories and Measuring and Test Equipment – General Requirements,” ANSI/NCSL Z540-1-1994 (R2002). Available from the NCSLI Business Office, Boulder, CO, 80301, USA. [2] “General Requirements for the Competence of Testing and Calibration Laboratories,” ANS/ISO/IEC 17025:2005. Available from the NCSLI Business Office, Boulder, CO, 80301, USA. [3] “U.S. Guide to the Expression of Uncertainty in Measurement,”ANSI/NCSL Z540-2-1997 (R2002). Available from the NCSLI Business Office, Boulder, CO, 80301, USA. [4] “Establishment & Adjustment of Calibration Intervals,” NCSLI RP-1. Available from NCSLI Business Office, Boulder, CO, 80301, USA. www.ncsli.org SMART/CMS Enterprise Calibration Management Software YOUR GOAL, OUR SOLUTION Now available on SQL Server 2005 and Oracle 10GR2! AssetSmart by PMSC 2800 28th Street, Suite 109 Santa Monica, CA 90405 USA 310.450.2566 www.assetsmart.com info@assetsmart.com Scalable from laptop to mega-enterprise Compliance Assurance for ANSI/NCSL Z540, ISO 9000, SAE AS9000, ISO 17025 and FDA 21 CFR Part 11 Automated Tracking of OOT/SOOT notices Integration Tools connect to calibrators and data acquisition systems User Customizable screens Integrated Equipment Management module Tool Crib and Spares Inventory module TECHNICAL PAPERS Uncertainties Related to Thermal Expansion in Dimensional Metrology Te d D o i ro n A b s t r a c t : Thermal expansion effects are very important in dimensional metrology. In this paper a measurement model, and associated equations, are developed for the case of a one-dimensional measurement of a steel test gage using a measuring machine and master gage. After presenting the uncertainty components for this measurement, several example measurement situations with different levels of temperature control are calculated and discussed. For each situation, the magnitude of the different sources of uncertainty are compared in order to rationally allocate resources to improve the overall measurement uncertainty. 1. I nt ro d u c t i o n In dimensional measurement the uncertainty is often dominated by the effects of thermal expansion. [1] This paper discusses these effects, their sources, and the methods used to determine the uncertainty components. In an extended example, the thermal uncertainty components for the measurement of a steel gage on a one-dimensional universal length measuring machine (ULMM) is derived for different levels of laboratory temperature control and measurement. By increasing the knowledge of the temperature of the instrument and gages, the uncertainty of the measurement is dramatically lowered. and G(tg) = [1 + αg(tg – 20)] G20 . (3) We also have the scale readout, S(ts). What we would like, of course, is the actual difference in length between M(tm) and G(tg). However, the scale reading is not correct because the scale also changes with temperature. Suppose the temperature is above 20 ºC. The scale is now longer, and the distance we measure will seem smaller than it really is. We thus have to correct the scale reading by enlarging it in proportion to the thermal expansion. Thus, the actual length difference between the gage and master is Smeas, 2 . Th e M odel In order to make a length measurement, we must take the actual (4) Smeas = [1+ αs(t s – 20)] S20 . measured values and calculate what the length would be at exactly 20 ºC (68 ºF). In the most general case, we will have a Putting these together, we get: measuring machine with a scale (S), a master gage (M), and a test gage (G). To make the corrections we must have the tem[1+ αs(ts – 20)] S2 0 = [1 + αg(tg – 20)] G2(5) 0 – [1 + αm(tm – 20)] perature and coefficients of thermal expansion (CTEs) of each. [1+ αs(ts – 20)] S2 0 = [1 + αg(tg – 20)] G2 0 – [1+ αm(tm – 20)] M20 . The actual measurement equation is: S(ts) = G(tg) – M(tm) , (1) where M is the length of the master gage, G is the length of the test gage, and S is the apparent difference in the scale readings for M and G. Each of these depends on their temperature, tm, tg, and ts, respectively. If we denote each CTE with α, and the scale is calibrated to be correct at 20 ºC, we find that: M(t m) = [1+ α m(t m – 20)] M 20 Ted Doiron Engineering Metrology Group National Institute of Standards and Technology Gaithersburg, MD 20899-8211 USA Email: doiron@nist.gov 32 | MEASURE (2) Now we make a small replacement to make the equation easier to handle; we use the Greek V to stand for (t – 20). We then get: (1+ αsVs) S20 = (1+ αgVg) G20 – (1+ αmVm) M2 0 . (6) If we solve for the length of the gage, G20, we get: G20 = (1+ α mV m)M20 + (1+ α sVs )S20 . (1+ α g Vg ) (7) Now, we can simplify this a bit by noting that the second term in the denominator is much smaller than 1. We can expand (1 + αgVg)–1 in a Taylor series, and keep only the first two terms, –1 (1 + αgVg) ≈ ( 1 – αgVg) . (8) www.ncsli.org TECHNICAL PAPERS Then we get as our equation: G20 ≈ (1 + αsVs)(1 – αgVg) S20 + (1 + αmVm)(1 – αgVg) M20 . (9) Again, since all the αV terms are much smaller than 1, we can multiply out the right hand side and ignore all of the terms of order (αV)2 and higher. We are left with the final equation for the length of the gage: G20 ≈ (1 + αsVs – αgVg) S20 + (1 + αmVm – αgVg) M20 . (10) How good is this equation? Let us take an extreme case of a measurement at 0 ºC on plastic. Plastics have very large CTEs, some nearly 100 #10-6/ºC. Thus, the term αV ≈ 100 × 10–6/ºC × 20 ºC × L ≈ 0.002 L . (11) The second order terms, being (αV)2 ; 0.000 004 L, are about 500 times smaller. For most laboratory conditions, the ratio is much larger, and therefore these second order terms are negligible. Thus, to make thermal corrections to a general measurement, we need a number of quantities that are shown in Table 1. All of these quantities have some uncertainty, u, associated with them. The thermal error terms are shown in Table 2. There are also the uncertainties of the scale reading and the length of the master gage to include in the overall uncertainty budget. Since we are focusing on sources associated with thermal expansion, however, we will not say much about these. Usually the scale reading uncertainty comes from the certificate or the manufacturer’s specification if it is certified to its specification, rather than being calibrated. If the correction from the master gage calibration certificate is used, the scale reading uncertainty is taken from the calibration laboratory’s stated uncertainty. If the calibration only certifies the gage to an accuracy class or grade, the width of the class or grade tolerance is taken as a rectangular distribution. [2] The rest of the quantities are more difficult to estimate; you actually have to think a bit. In some cases the CTEs can be determined to some precision from calibration. At NIST we have calibrations of the CTEs of our gages, either in house or αs CTE of the scale of the measurement instrument αm CTE of the master gage αg CTE of the test gage ts from calibration reports from the manufacturer. In these cases the uncertainty in the CTE can be quite small, 0.1 #10-6/ºC or better. Without such detailed information you must use whatever information you can get. The range of CTE for “steel” is quite large, but there are only a few gage block steels, and the range of CTE for these is somewhat smaller. The Standards for gage blocks [3] usually require that the CTE of a steel gage block is 11.5 #10-6/ºC with a tolerance of ±1 #10-6/ºC. Other materials, such as tungsten carbide, chrome carbide, ceramic, etc., have no specification and most people will assume ±10 % as the uncertainty. [4] The temperatures are more complicated, still. Some notes: 1. If you only have one thermometer, and it is used only to monitor the room, then you must use the daily variation in the room temperature as the uncertainty for everything. This is a very large number, but uncertainty is a measure of your ignorance of the measurement and if you don’t measure something you are pretty ignorant. 2. If your thermometer is calibrated you still cannot automatically use the uncertainty on the certificate as your uncertainty. Many thermometers, particularly low cost thermistors, drift over their calibration cycle. I have been in many laboratories that have thermometers with certificates that say the uncertainty in the thermometer calibration is 0.01 ºC, but when examining the calibration history, I find that the thermometer is adjusted by 0.05 ºC to 0.10 ºC or more each time it is recalibrated. In general, the historical variation in the thermometer is the acceptable uncertainty. 3. As in most “meets manufacturer’s specification” types of calibrations, if the thermometer is not adjusted (see note 2 above), you can take the specification as a rectangular distribution. To demonstrate the effects of thermally related sources of uncertainty, the uncertainty budget for a single measurement is analyzed. The first example is for a laboratory with only the most basic knowledge of the environment, and succeeding examples illustrate how the uncertainty can be lowered by changing the level of temperature measurement and control. The example is for the comparison of a test ring gage to a master ring gage using a ULMM. [5] The master ring gage is calibrated by an accredited laboratory with an uncertainty of 0.5 µm. Uncertainty in CTE of the scale u(αs) Vs S(ts) Uncertainty in CTE of the test gage u(αg) Vg [S(ts) + M20] temperature of the scale Uncertainty in CTE of the master gage u(αm) Vm M20 tm temperature of the master gage Uncertainty in scale temperature u(Vs) αs S(ts) tg temperature of the test gage Uncertainty in test gage temperature u(Vg) αg [S(ts) + M20] Uncertainty in master gage temperature u(Vm) αm M20 M20 calibrated length of the master gage S(t s) scale reading of the measurement instrument Ta bl e 1. Quantities needed in order to make thermal corrections to the measurement. Vol. 1 No. 4 • December 2006 Tab le 2. Thermal error terms for the measurement. MEASURE | 33 TECHNICAL PAPERS 3. E x a m p l e 1 : 10 0 mm cus tom er ri ng gag e c a li b ra te d usi ng a 100 mm m a ste r ri ng o n a l on g ra ng e ULMM . L abo r a to r y h as o n e t h er m o me te r t o m o n i t o r ro o m . Master Gage 100 mm Ring Gage Uncertainty 500 nm (k = 2) ULMM Specification 0.2 µm + 0.5 #10–6 L “Accuracy Specification” (Rectangular) Test Gage Material: Steel CTE = 12#10–6/ºC Uncertainty 10 % (Rectangular) Master Gage Material: Steel CTE = 12 #10–6/ºC Uncertainty 10 % (Rectangular) The typical uncertainty budget for this measurement equation is shown in Table 3. Scale Material: Glass Uncertainty 10 % (Rectangular) CTE = 7#10–6/ºC In addition: •Room temperature variation is ±1 ºC. Ta bl e 3. The values and uncertainties for the measurement in example 1. Since we only have one thermometer and it is used to determine some sort of room average temperaStd ture, we are hard pressed to say Unc. Uncertainty Std. Unc. Sensitivity Standard (µm) / Range of Factor Coeff. Uncertainty Source Dist. we know the temperature of anything to better than 1 ºC. So, Test Gage 1 ºC Rect. 0.58 ºC 12#10–6 L/ºC 0.69 6.9#10–6 L Temp. (Steel) we will take the uncertainty in all of the temperatures as a rectanMaster Gage 1 ºC Rect. 0.58 ºC 12#10–6 L/ºC 0.69 6.9#10–6 L Temp. (Steel) gular distribution of ±1 ºC. Note that if we were to measure the Scale Temp. 1 ºC Rect. 0.58 ºC 7#10–6 L/ºC 0.40 4.0#10–6 L (Glass) temperatures with the single thermometer, the uncertainties CTE (Scale) Rect. 0.5 ºC 0.020 0.7#10–6/ºC 0.40#10–6/ºC 0.20#10–6 L would be correlated and the CTE analysis would be more compliRect. 0.5 ºC 0.035 1.2#10–6/ºC 0.70#10–6/ºC 0.35#10–6 L (Master Gage) cated. Here, however, we are not CTE (Test Gage) 1.2#10–6/ºC Rect. 0.5 ºC 0.035 0.70#10–6/ºC 0.35#10–6 L using the thermometer to measure the actual temperatures Length of of the scale and gages, just to set 0.50 µm Normal 0.250 µm 1 0.250 µm 0.25 Master Gage limits on their variations. Scale We also have to estimate the 0.25 µm Rect. 0.150 µm 1 0.150 µm 0.15 Specification value of the temperature difference between the scale and gages Combined Standard Uncertainty 1.10 and 20 °C. In a typical room the Expanded Uncertainty (k = 2) 2.20 temperature change is roughly linear with time, so the average difference between 20 °C and Ta bl e 4. Uncertainty calculation for example 1. the scale and gages is 0.5 °C. It can be argued that this is an over estimate because the gages and ULMM act like low pass filters on the room air temperature, so Standard Std. Unc. Ratio to that the variations are more like sine waves than saw-tooth Uncertainty Squared Largest Source waves. This calculation is too complex for most laboratories and the changes are small compared to other sources of uncertainty. Test Gage Temp. (Steel) 690 nm 476,100 1 Thus, we will use 0.5 °C. The calculation of the uncertainty comMaster Gage Temp. (Steel) 690 nm 476,100 1 ponents is shown in Table 4. Scale Temp. (Glass) 400 nm 160,000 0.34 Let’s examine the biggest parts of this uncertainty. If we compare them through their variances (standard uncertainty CTE (Scale) 20 nm 400 0.001 squared), which is how they enter into the combined standard uncertainty, we get the results shown in Table 5. CTE (Master Gage) 35 nm 1,225 0.003 Since any source of uncertainty that is less than 1/3 to 1/4 of CTE (Test Gage) 35 nm 1,225 0.003 the largest component does not significantly contribute to the combined standard uncertainty, we see that there are three Length of Master Gage 250 nm 62,500 0.131 sources of uncertainty that dominate our measurement (highScale Specification 150 nm 22,500 0.047 lighted). Since all of these are determined by the temperature measurement, we can see that we need a better thermometer. To SUM 1,200,050 reduce our uncertainty, the laboratory buys a portable thermometer that can be positioned on or near the gages when they are Table 5. Comparison of the relative sizes of each uncertainty being measured; the new thermometer has a specification of 0.1°C. source for example 1 based on the ratio of variances. 34 | MEASURE www.ncsli.org TECHNICAL PAPERS Master Gage 100 mm Ring Gage Uncertainty 500 nm (k = 2) ULMM Specification 0.2 µm + 0.5 #10–6 L “Accuracy Specification” (Rectangular) Test Gage Material: Steel CTE = 12#10–6/ºC Uncertainty 10 % (Rectangular) 4. E x a m p l e 2 : 10 0 m m te st ri ng ga g e c a li b ra te d u si n g a 1 0 0 mm m as t er r in g o n a l o ng ra n ge UL M M. La b o r a to ry h a s p o r t a b l e t h e r m o m e t e r. The typical uncertainty budget for this measurement equation is shown in –6 Table 6. Scale Material: Glass Uncertainty 10 % (Rectangular) CTE = 7#10 /ºC In addition: • Test Gage and Master Gage temTa bl e 6. The values and uncertainties for the measurement in example 2. peratures are measured and corrected for using a digital thermometer; its uncertainty Std specification is ±0.1 ºC. Unc. Uncertainty Std. Unc. Sensitivity Standard (µm) / Range of Factor Coeff. Uncertainty Source Dist. • Average temperature during measurements = 20.45 ºC. Test Gage 0.1 ºC Rect. 0.058 ºC 12#10–6 L/ºC 0.7#10–6 L 0.069 Temp. (Steel) • Scale temperature cannot be measured. Master Gage 0.1 ºC Rect. 0.058 ºC 12#10–6 L/ºC 0.7#10–6 L 0.069 Temp. (Steel) • Room temperature variaScale Temp. tion is ±1 ºC. 1.0 ºC Rect. 0.58 ºC 7#10–6 L/ºC 4.0#10–6 L 0.400 (Glass) The calculation of the uncertainty components for this sitCTE (Scale) 0.7#10–6/ºC Rect. 0.40#10–6/ºC 0.5 ºC 0.20#10–6 L 0.020 uation is shown in Table 7. CTE –6 –6 –6 Let’s examine the biggest 1.2#10 /ºC Rect. 0.70#10 /ºC 0.45 ºC 0.31#10 L 0.031 (Master Gage) components of this uncerCTE (Test Gage) 1.2#10–6/ºC Rect. 0.70#10–6/ºC 0.45 ºC 0.31#10–6 L 0.031 tainty. If we compare them by their variances (standard Length of 0.50 µm Normal 0.25 µm 1 0.250 µm 0.250 uncertainty squared), which is Master Gage how they enter into the comScale 0.25 µm Rect. 0.15 µm 1 0.150 µm 0.150 bined standard uncertainty, we Specification get the results in Table 8. We have now reduced our Combined Standard Uncertainty 0.51 uncertainty quite a bit, but we Expanded Uncertainty (k = 2) 1.02 still have an expanded uncertainty that is not very good (1.02 µm). It is obvious where Ta bl e 7. Uncertainty calculation for example 2. we next need to improve our process: the scale. One way is to make the scale out of a material that has a very low CTE, like fused silica, or more engineered materials like Zerodur or ULE Zero Expansion Std. Unc. Ratio to Standard Glass. Fused silica has a CTE of 0.5#10-6/ºC, which is conSquared Largest Uncertainty Source siderably less than glass. There are a number of engineered Test Gage Temp. (Steel) 69 nm 4,761 0.03 materials with a CTE less than 0.1#10-6/ºC. All of these will Master Gage Temp. (Steel) 69 nm 4,761 0.03 help. Another way is to put a thermometer on the scale, and make corrections. Scale Temp. (Glass) 400 nm 160,000 1.0 Another way that should help, but is problematic at times, is to have the scale be steel and measure steel parts. If the CTE (Scale) 20 nm 400 0.003 scale and parts are the same temperature, and they usually CTE (Master Gage) 31 nm 961 0.006 are closer in temperature to each other than the room variCTE (Test Gage) 31 nm 961 0.006 ation, you could get a lower differential thermal expansion correction and therefore lower the uncertainty some. UnforLength of Master Gage 250 nm 62,500 0.39 tunately many instruments have the scale in a closed housing and it is difficult to document the temperature difference. Scale Specification 150 nm 22,500 0.14 Master Gage Material: Steel CTE = 12 #10–6/ºC SUM Uncertainty 10 % (Rectangular) 256,844 Ta bl e 8. Comparison of the relative sizes of each uncertainty source for example 2 based on the ratio of variances. Vol. 1 No. 4 • December 2006 MEASURE | 35 TECHNICAL PAPERS 5. E x a m p l e 3 : 1 0 0 mm t est ri n g ga ge c al ib ra t e d u sin g a 1 0 0 mm ma st e r r i ng o n a lo n g r a n g e ULM M tha t has a lo w t her mal c o e ff i c i e n t m a t e r ia l a s t h e scal e. Labo r ato ry has po r tabl e t h e r m o m e t e r. Master Gage 100 mm Ring Gage Uncertainty 500 nm (k = 2) ULMM Specification 0.2 µm + 0.5 #10–6 L “Accuracy Specification” (Rectangular) Test Gage Material: Steel CTE = 12#10–6/ºC Uncertainty 10 % (Rectangular) Master Gage Material: Steel CTE = 12 #10–6/ºC Uncertainty 10 % (Rectangular) Scale Material: Low CTE CTE = 0.1#10–6/ºC Uncertainty 10 % (Rectangular) The typical uncertainty budget for this measurement equation is shown Ta bl e 9. The values and uncertainties for the measurement in example 3. in Table 9. In addition: • Test Gage and Master Gage Std temperatures are measured Unc. Uncertainty Std. Unc. Sensitivity Standard (µm) / Range of Factor Coeff. Uncertainty Source Dist. and corrected for using a Test Gage digital thermometer; uncer0.1 ºC Rect. 0.058 ºC 12#10–6 L/ºC 0.69#10–6 L 69 Temp. (Steel) tainty specification is ±0.1 ºC. Master Gage • Average temperature during 0.1 ºC Rect. 0.058 ºC 12#10–6 L/ºC 0.69#10–6 L 69 Temp. (Steel) measurements = 20.45 ºC. Scale Temp. • Scale temperature cannot be 1.0 ºC Rect. 0.58 ºC 0.1#10–6 L/ºC 0.057#10–6 L 6 (Low CTE) measured. • Room temperature variation CTE (Scale) 0.01#10–6/ºC Rect. 0.006#10–6/ºC 0.5 ºC 0.003#10–6 L 0.3 is ±1 ºC. CTE –6 –6 –6 1.2#10 /ºC Rect. 0.70#10 /ºC 0.45 ºC 0.31#10 L 31 (Master Gage) For our example, we will use a fictional engineered material CTE (Test Gage) 1.2#10–6/ºC Rect. 0.70#10–6/ºC 0.45 ºC 0.31#10–6 L 31 that has a known thermal Length of expansion of 0.1 #10-6/ºC ± 500 nm Normal 250 nm 1 50 nm 250 Master Gage 0.01#10–6/ºC. The calculation Scale of the uncertainty components 0.25 µm Rect. 150 nm 1 150 nm 150 Specification for this example is shown in Table 10. Combined Standard Uncertainty 310 We now have only one large Expanded Uncertainty (k = 2) 620 component we can change, the uncertainty of the master ring. Suppose we send the ring gage Tabl e 10. Uncertainty calculation for example 3. to NIST for calibration. At NIST the ring is calibrated on our M48 coordinate measuring corrected for using a digital thermometer; uncertainty specmachine in a laboratory that is temperature controlled to ±0.01 ification is ±0.1 ºC. °C. The one directional repeatability of the M48 is less than 0.03 • Average temperature during measurements = 20.45 ºC. µm, and our long term reproducibility studies shows a calibra• Scale temperature cannot be measured. tion uncertainty for a 100 mm ring gage to be 0.12 µm. • Room temperature variation is ±0.1 ºC. With the development of long range instruments like the The calculation of the uncertainty components for this example ULMM, the need for matching the master gage to the test gage is shown in Table 12. is no longer necessary. This opens up the opportunity to use the At this point, improvements become harder because there are calibration services at NIST for laboratories that cannot afford a number of thermal components of about the same size, and the calibration of a whole gage block set. For nearly all size ring the largest is from the ULMM. Fixing only one will not provide gages, only a few masters are needed. much improvement in the expanded uncertainty. Basically, for most calibration laboratories, this is the end of 6. E x a m p l e 4 : the road. If you look at industrial interlaboratory test data for ring 1 0 0 mm t e st rin g ga ge ca l ib ra t e d u s in g a 1 0 0 mm ma st e r or plug gage measurements, you see things like those in Fig. 1. [6] ri n g o n a lo n g r an ge U L M M wh ic h h a s a sc a le ma d e of a The results in Fig. 1 are typical of a round robin for randomly l o w C T E m a t e r i a l . L a b o r a t o r y h a s p o r t a b l e t h e r m o m e t e r. selected calibration laboratories. In this round robin, there were T h e m a s t e r r i n g i s c a l i b r a t ed a t N I ST. two rings and the deviations are the differences from the NIST calibration. The standard deviation between the laboratories The typical uncertainty budget for this measurement equation was, in all cases, around 0.4 µm. If we take this as a rough estiis shown in Table 11. mate of the standard uncertainty of the group, we would get a In addition: k = 2 expanded uncertainty of around 0.8 µm. According to our • Test Gage and Master Gage temperatures are measured and 36 | MEASURE www.ncsli.org TECHNICAL PAPERS Master Gage 100 mm Ring Gage Uncertainty 120 nm (k = 2) ULMM Specification 0.2 µm + 0.5 #10–6 L “Accuracy Specification” (Rectangular) Test Gage Material: Steel CTE = 12#10–6/ºC Uncertainty 10 % (Rectangular) Master Gage Material: Steel CTE = 12 #10–6/ºC Uncertainty 1 % (Rectangular) Scale Material: ULE-like CTE = 0.1#10–6/ºC Uncertainty 10 % (Rectangular) uncertainty budget in Example 3, this is about what we would expect for measurements made in an “average” environment (±1 ºC) with decent thermometers (±0.1 ºC) using a ULMM with a low expansion material scale. 7. S u mma r y Ta bl e 11. The values and uncertainties for the measurement in example 4. Source Uncertainty / Range Dist. Std. Unc. of Factor Sensitivity Coeff. Standard Uncertainty Std Unc. (µm) Test Gage Temp. (Steel) 0.1 ºC Rect. 0.058 ºC 12#10–6 L/ºC 0.69#10–6 L 69 Master Gage Temp. (Steel) 0.1 ºC Rect. 0.058 ºC 12#10–6 L/ºC 0.69#10–6 L 69 Scale Temp. (Low CTE) 0.1 ºC Rect. 0.058 ºC 0.1#10–6 L/ºC 0.006#10–6 L 0.6 CTE (Scale) 0.01#10–6/ºC Rect. 0.006#10–6/ºC 0.5 ºC 0.003#10–6 L 0.3 CTE (Master Gage) 1.2#10–6/ºC Rect. 0.70#10–6/ºC 0.45 ºC 0.31#10–6 L 31 CTE (Test Gage) 1.2#10–6/ºC Rect. 0.70#10–6/ºC 0.45 ºC 0.31#10–6 L 31 Length of Master Gage 120 nm Normal 60 nm 1 60 nm 60 Scale Specification 0.25 µm Rect. 150 nm 1 150 nm 150 Thermal expansion is a critical part of uncertainties for dimensional measurements. A fairly simple analysis of the uncertainty components can be used to decide how to rationally allocate resources to obtain adequate measurement performance. 8. Acknowledgment I would like to thank Richard Pettit and the anonymous reviewer for pointing out a remarkable number of numerical errors in this paper. The paper is much stronger for their efforts. 9. R ef er e n c e s [1] Strangely, few metrology books even mention thermal expansion. A survey of over 50 Expanded Uncertainty (k = 2) 386 books on dimensional metrology or inspection revealed only two with mentions of thermal Ta bl e 12. Uncertainty calculation for example 4. expansion, and those had only one paragraph each. The best source for information on thermal expansion is ASME/ANSI B89.6.2, Temperature and Humidity Environment for Dimen1.5 sional. It contains details on all of the concepts used in this paper. [2] International Organization for Standardization (ISO), Guide to the 1 Expression of Uncertainty in Measurement, Geneva, Switzerland, 1993. 0.5 [3] American Society for Mechanical Engineering, ASME/ANSI 0 B89.1.9, Gage Blocks, New York, NY, 2002. 1 3 5 7 9 1 1 1 3 1 5 1 7 19 21 2 3 2 5 2 7 2 9 3 1 3 3 [4] There are a large number of reference books that list the CTE of -0 . 5 various materials. Unfortunately the uncertainty is seldom reported, and the CTEs reported in most sources are averages over -1 large ranges of temperature, which increases the uncertainty at 20 °C. The 10 % uncertainty used in this paper is the consensus -1 . 5 value used by experts in the field. Lab [5] How a 1D measuring machine came to be known as a “universal” F i g u re 1. Deviation from the NIST calibration value for a 63.1825 mm measuring machine is not known, but probably results from the (2.4875 in.) ring gage as measured by 34 laboratories. The range of the data is 2.0 µm, with a standard deviation of 0.4 µm. fact that it can be used to measure both internal (ring) and external (plug) dimensions. A machine that measures in three dimensions is occasionally called a “universal measuring machine,” but the most common term is “coordinate measuring machine.” [6] Private correspondence. 193 Deviation from NIST (µm) Combined Standard Uncertainty Vol. 1 No. 4 • December 2006 MEASURE | 37 TECHNICAL PAPERS A Theory for RF and Microwave Scalar Reflectometer Errors 1 Robert D. Moyer A b s t r a c t : Scalar reflectometers afford a relatively inexpensive means to measure reflection coefficient magnitudes at RF and microwave frequencies. Unfortunately, the measurements include errors that arise from vectorial imperfections within the reflectometer hardware. While a vector network analyzer can help correct for the imperfections, a scalar analyzer has only limited capability to do so, and there is often confusion about the extent of the corrections that can be made. This paper provides a careful analysis of rf and microwave scalar reflectometers and discusses two common ways to initialize them. The results (1) show the advantages, and remaining weaknesses, of the open/short method for initializing a 4-port reflectometer, (2) demonstrate the irrelevance of detector reflection coefficients, (3) reveal the advantages of 4-port over 3-port reflectometers, and (4) present expressions showing how the worst-case errors in scalar reflectometer measurements vary, depending on the properties of both the reflectometer and the device under test. 1 . In t ro d u c t i o n RF and microwave reflectometers are often used in conjunction with scalar receivers to make moderately accurate reflection coefficient measurements. Measurements using a scalar reflectometer are subject to errors that arise from vectorial imperfections in the reflectometer hardware. In this paper, a careful analysis of both a 3-port and 4-port reflectometer is presented which is then used to demonstrate the advantages of a 4-port reflectometer over a 3-port reflectometer. The following five sections discuss the theory involved in scalar reflectometer measurements and includes two ways to initialize the reflectometer, as well as an analysis of the worst-case errors associated with a well-initialized scalar 4-port reflectometer. Robert D. Moyer Primary Standards Laboratory Sandia National Laboratories P.O. Box 5800 Albuquerque, NM 87185-0665 USA Email: rdmoyer@sandia.gov 38 | MEASURE 2. R e p r e s e n t a t i o n o f R e f l e c t o m e t e r s a n d N o t a t i o n Figures 1a and 1b show schematic representations of 4-port reflectometers based on a directional coupler and a power splitter–directional bridge combination, respectively. For analysis purposes, Fig. 1c shows the signal flow diagram representation of a 4-port reflectometer. A signal generator, whose internal generator reflection coefficient is Γg, drives port 1 of the reflectometer. The initialization and unknown devices are connected to port 2, the test port. The true reflection coefficient of any device connected to test port 2 is denoted as z. The signal measured at port 3 is primarily proportional to the signal emergent from test port 2 of the reflectometer (i.e., incident on the device connected to port 2) while the signal measured at port 4 is primarily proportional to the signal reflected from the device connected to test port 2. Thus, the ratio of the latter signal to the former signal is approximately proportional to the reflection 1 Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. www.ncsli.org TECHNICAL PAPERS z 3 4 2 1 b a 2 2 s22 s23 s21 F i g u re 1a. Directional coupler. bg s32 b a1 3 s31 1 Γg s11 b1 3 s34 s42 s24 s12 ρ3 → s33 a s13 3 4 s14 s43 s41 s44 2 F i g u re 1b. Splitter – directional bridge. a4 b4 ρ4 ↓ F i g u re 1c. 4-port signal flow diagram. coefficient of the device connected to port 2. The ratio of the traveling wave voltage emergent from port 4, b4, to the traveling wave voltage emergent from the generator, bg, may be z ( S 21 S 42 − S 22 S 41 ) + S 41 b written by inspection of the signal flow graph (see Section 2.18 , (3) wm = 4 = b3 z ( S 21 S 32 − S 22 S 31 ) + S 31 of Ref. [1]) as: S S − S or S 41 S 41 22 z 21 42 + 1− Γ S 1− Γ S 1− z S + S zS S b4 g 11 41 ( 22 ) 21 42 S 42+−e S 22 S 41 ) S 31 + S 41 S 31 qz + r b 4g 11z ( S=21hz = = (1) , (4) w = = bg 1− Γg S11 − zS 22 + Γg S11 z S 22 − Γg S 21 zS12 Γg S11 S 22 − Γg S 21 S12m − Sb22 = fz +1 sz +1 S S 3 z +1 −z S 22 − 21 32 +1 1− Γg S11 S 31 S S −S S S 41 21 42 22 41 z + − z S 22 ) + S 21 zS 42 1− Γg S11 1− Γg S11 hz + e , where q, r and s are complex functions of the S-parameters of = = the reflectometer as indicated in Eq. (4). + Γg S11 z S 22 − Γg S 21 zS12 Γg S11 S 22 − Γg S 21 S12 − S 22 fz +1 z +1 For 3-port reflectometers, one uses Eq. (1) for the measured 1− Γg S11 ratio because, using the notation of Fig. 1, port 3 is not present; for 4-port reflectometers, one uses Eq. (4). where: h, e and f are functions of Γg and the S-parameters of the reflec3 . A G o o d Wa y t o I n i t i a l i z e S c a l a r R e f l e c t o m e t e r s tometer as suggested in Eq. (1); ρ3 and ρ4 (in Fig. 1c) are the reflection coefficients of the detecIf a vector receiver is available to measure the complex ratios tors on ports 3 and 4, respectively; and and if the complex S-parameters of the reflectometer are known, then very accurate reflection coefficient measurements ρ3 and ρ4 were set to 0 during the derivation of Eq. (1) in anticcan be made. It is, however, much less expensive to use scalar ipation of the result of section 4. (There it is shown that the sensors, which respond only to amplitudes, on a reflectometer. values of ρ3 and ρ4 are arbitrary when the reflectometer is Unfortunately, accuracy decreases when scalar detectors are initialized by the preferred technique described in section used on a 4-port reflectometer. This section discusses a way to 3.) Equations (1) through (4) would be much more cummitigate the loss of accuracy through use of a well-known inibersome if ρ3 ) 0 and ρ4 ) 0. tialization procedure. Similarly, replacing 4’s by 3’s in Eq. (1): Equations (1) and (4) show that both b4/bg (measured on a 3-port reflectometer) and b4/b3 (measured on a 4-port reflecS S −S S S 31 22 31 z 21 32 + tometer) are given by linear fractional expressions. Because of b3 1− Γg S11 1− Γg S11 . (2) this, the discussion in this section applies to both 3- and 4-port = bg Γg S11 S 22 − Γg S 21 S12 − S 22 reflectometers. For convenience, however, we will use the notaz +1 1− Γ S tion for the 4-port reflectometer. g 11 If one simply connects a device under test (DUT) to the test port of a scalar reflectometer and measures it, the scalar reflecTaking the ratio of the preceding two equations gives the meastometer produces (at least internally) a value, |wm|, which is ured ratio, wm, of the emergent traveling wave voltage from port 4 to that at port 3 as: assumed to obey the simple model: Vol. 1 No. 4 • December 2006 MEASURE | 39 TECHNICAL PAPERS wm = Q wt , | wm | where wt is the true reflection coefficient of the DUT and Q is a scalar constant determined by the scalar reflectometer hardware. In reality, |wm| obeys the more complex model of Eq. (5), which is merely the magnitude of Eq. (4): | wm | = | b 4 | | qz + r | . = | b3 | | s z +1| (5) or | wm | ws wo | q z + r | | se − j φ +1| | −se − j φ +1| , | sz +1| | qe − j φ + r | | −qe − j φ + r | = | q z + r | | −s 2 e − j 2 φ +1| , | sz +1| | −q 2 e − j 2 φ + r 2 | or | −s 2 e − j 2 φ +1| If r = s = 0 (as in a perfect reflectometer), Eq. (5) collapses to the simple model. Nevertheless, even in a perfect reflectometer, the value of q differs significantly from unity as shown by the coefficient of z in the numerator of Eq. (4). Equation (5) therefore provides a poor estimate of |wt|; the reflectometer needs to be “initialized” in order to obtain a better estimate of |wt|. First, a poor way to initialize the reflectometer is described in the following three steps: Step 1A: Measure an open circuit, i.e., z = e–jφ, at test port 2 which gives: | qe − j φ + r | . | wo | = | se − j φ +1| ws wo = (6) | wm | | −e − j 2 φ + ( r q ) | 2 = | −s 2 e − j 2 φ +1| | −e − j 2 φ + ( r q ) | | sz +1| ws wo or z + (r q ) | wm | ws wo = | q'' z + r" | . | s z +1| 2 . (10) (11) where q” is the coefficient of z in the numerator of Eq. (10) and r” is the final term in that numerator. From Eq. (4), it follows that: S 41 S 41 r , = ≈ q S 21 S 42 − S 22 S 41 S 21 S 42 (12) where the approximation holds because |S22S41| << |S21S42| Step 2A: Divide Eq. (5), which results from measuring the unknown device, by Eq. (6) to obtain (after re-arranging): for both circuits shown in Fig. 1. Furthermore, |r/q|2 ; − jφ |d/S |2 << 1 since the coupler or directional bridge will have | se +1| | se − j φ +1| z − jφ + ( r q ) − j φ 21 − jφ high directivity, i.e. very small |d| = |S41/S42|, if it is used in a | e + (r q ) | | e + (r q ) | | wm | | q z + r | | se +1| (7) = = reflectometer application. Also, as previously mentioned, |s|2 | sz +1| | sz +1| | qe − j φ + r | | wo | <<1 so the coefficient of z in the numerator of Eq. (10) reduces to a value very near unity. − jφ − jφ | se +1| | se +1| z − jφ + (r q ) − j φ Equations (10) and (11) therefore show that | e + (r q ) | | e + (r q ) | | wm | | q z + r | | se − j φ +1| , = = | wm | | sz +1| | sz +1| | qe − j φ + r | | wo | ws wo or | wm | | zq'+ r ' | . = | wo | | s z +1| (8) Step 3A: Equation (8) shows that the estimator |wm/wo| equals |z| if r' = s = 0 and |q'| = 1. Unfortunately, r' and s are often large enough to drive |q'| away from unity because q' has first order dependence on r and s as seen in Eq. (7). A preferred, well-known, procedure for initialization is given in step 1B through step 3B which follow: Step 1B: Measure the unknown and open circuit to get Eqs. (5) and (6), respectively, as shown above. Step 2B: Measure an offset short circuit, i.e., z = e–j(π+φ ), which is π radians out of phase with the open circuit, to get: | ws | = | qe − j (π + φ ) + r | | −qe − j φ + r | . = | se − j (π + φ ) +1| | −se − j φ +1| (9) Note: A short/open pair which very closely provides the π radian phase difference is available from most manufacturers of scalar reflectometers. See Appendix A. Step 3B: Divide Eq. (5) by the square root of the product of Eqs. (6) and (9) to obtain: 40 | MEASURE is a better estimator because q" has only second order dependence on the small, but inevitably present, r and s parameters! |q"| will therefore more closely approximate unity than does |q'| as defined by Eqs. (7) and (8). Steps 1B through 3B therefore define a preferred technique for initializing a scalar reflectometer. The open-short initialization technique gives a value of q" that is very near unity. While this is an advantage, it is unfortunately the only benefit that results from use of the open-short initialization technique. First, notice that the denominators of the right-hand-sides of Eqs. (5), (8) and (11) are identical, namely |s z + 1|; hence the harmful effects of s are the same in all three equations. Engen [2] has shown that Γge = –s is the effective generator reflection coefficient of the 4-port reflectometer. Results from Eq. (11) therefore contain unmitigated errors due to Γge. Equation (4) may be rewritten as: r qz + q . wm = sz +1 (4a) Clearly, the numerator of the expression for wm includes an undesirable component, r/q, which is a function only of reflectometer properties and is independent of z. Again, from Eq. (12), www.ncsli.org TECHNICAL PAPERS r = q S 41 S 42 S 21 − S 22 , S 41 S 42 (12a) where S41/S42 is the directivity as observed at port 4 of the reflectometer. Equations (7) and (8) reveal that r'/q' = r/q while Eqs. (10 and (11) show that r"/q" = r/q. This shows that the preferred initialization does nothing to mitigate the effects of the directivity of the reflectometer. Thus, measurements of unknowns will still be in error due to (a) multiple reflections between the unknown and the effective generator reflection coefficient of the reflectometer, and (b) the directivity of the reflectometer. 4. I r re l e v a n c e o f D e t e c t o r R e f l e c t i o n C o e ff i c i e n t s Equations (1) through (4) involve the traveling wave voltages, while most detectors respond to power or the voltage squared. Furthermore, detectors always reflect a small portion of any signal incident upon them. However, the reflection coefficients (ρ3 and ρ4 in Fig. 1c) of the detectors do not influence the results from a scalar reflectometer that is initialized by the preferred technique described in the preceding section. Again, this is true for both 3- and 4-port reflectometers as shown in the following paragraphs. In practice, power, rather than bix, (i = 3 or 4; x = ‘m’ for unknown being measured, ‘s’ for short and ‘o’ for open measured during initialization) is measured at each sidearm port of a scalar reflectometer. As shown by Eq. (2.37) of Ref. [1], the power incident on detector 4 in Fig. 1c is given by |b4x|2 /Z0. Similarly, the power reflected from detector 4 is |b4xρ4|2 /Z0. Therefore, the power absorbed by detector 4 is P4x = |b4x|2 (1 – ρ42) / Z0 where: Pix is the power absorbed by detector i when voltage wave bix is incident on it; Z0 is the (real) characteristic impedance of the system; and ρi is the magnitude of the reflection coefficient of detector i. Consequently, P4x Z 0 b4x = . 1 − ρ 42 The ratio ( ) ) ( ) P4 x Z 0 1 − ρ 32 P4x 1 − ρ 32 b 4x = = 2 b3x 1 − ρ 4 P3x Z 0 1 − ρ 42 P3x ( ( ) (13) therefore follows. Using Eq. (13) and referring to Eqs. (4) and (11), the preferred estimator for a 4-port scalar reflectometer is given by: b4 m b3 m b4 s b4o b3 s b3o ( P4 m 1 − ρ 32 ) (1 − ρ ) P = P (1 − ρ ) P (1 − ρ ) (1 − ρ ) P (1 − ρ ) P 2 = 4 3m 2 4s 3 2 4 2 4o 3 2 3s 4 3o P4 m P3 m P4 s P3 s . P4 o P3o It is important to notice that parameters q, r and s in Eq. (4) are entirely independent of Γg, the reflection coefficient of the generator which drives the 4-port reflectometer. It therefore follows that the preferred estimator, see Eq. (11), for a 4-port scalar reflectometer is also independent of Γg. On the other hand, if a 3-port reflectometer is used, one must work with Eq. (1) because port 3 does not exist. In this case, the parameter f in Eq. (1) has three terms which depend on Γg ; the ΓgS21S12 term, in particular, can be large enough to cause significant errors in the measured reflection coefficient. Furthermore, it was pointed out in the last paragraph of section 3 that the parameter s (for a 4port reflectometer) is not diminished by use of the preferred initialization procedure. It is easily shown that the same is true of the parameter f for the 3-port reflectometer. The parameter f, and therefore the error in measurements from a 3-port reflectometer, changes whenever Γg (the generator reflection coefficient of the generator driving the 3-port reflectometer) changes. Based on these considerations, two significant advantages of the 4-port reflectometer are: 1. The performance of the 4-port reflectometer is independent of Γg, the reflection coefficient of the generator which drives it, while a 3-port reflectometer’s performance depends on the reflection coefficient of its driving generator. 2. If b3 and b4 are measured simultaneously, the 4-port reflectometer is immune to any changes in generator level that may occur while carrying out the initialization and DUT measurements. On the other hand, the 3-port reflectometer provides no mechanism for eliminating the effects of such changes. 6. Wo r s t - C a s e E r ro r s o n M e a s u r e m e n t s f ro m a We l l - I n i t i a l i z e d S c a l a r 4 - P o r t R e f l e c t o m e t e r Equation (11), as a real equation, specifies the preferred estimator from a scalar reflectometer; the magnitude bars are required because the scalar detectors cannot sense vector information. However, the error mechanisms in the scalar reflectometer are vector in nature. In order to characterize the errors in the scalar estimator, it is necessary to analyze the vector relationships obtained by removing the magnitude restraints from Eq. (11). Thus: wm q'' z + r " (15) = =w . sz +1 ws wo Equation (15) shows that the vector estimator w is a complex bilinear transformation of z, the true vector reflection coefficient of the device being measured. Inverting Eq. (15) gives: (14) This shows that the preferred estimator is independent of both detector reflection coefficients. Vol. 1 No. 4 • December 2006 5. A d v a n t a g e s o f a 4 - P o r t R e f l e c t o m e t e r Ov e r a 3-P o r t R e f l e ct o me t e r z= w −r" a"w + b " . = −s w + q" cw +1 (16) Therefore, if the locus of w is a circle at the origin of the complex plane, the locus of z will be another circle of radius R1 whose center lies at the tip of a vector C1. According to Eqs. MEASURE | 41 TECHNICAL PAPERS C z locus 1 R 1 |w| w locus F i g u re 2. A possible relationship between w and z loci. (2.153) and (2.154) from Kerns and Beatty [1], R1 and C1 are given by: | a"− b "c || w | 1− |cw| 2 (2.153) b "− a"c* | w | 2 , respectively. 1− | cw |2 (2.154) R1 = and C1 = Knowing that |a"| ~ 1, |b"| < 1 and |c| < 1 for a well-initialized 4-port scalar reflectometer, Eq. (2.153) shows that R1 will be approximately equal to |w| while Eq. (2.154) shows that |C1| will very likely exceed R1 for small values of |w|. Figure 2 depicts a possible relationship between the locus of w, which is centered at the origin, and the locus of z, which is centered at the tip of C1. The worst-case error at a given frequency ν, WCEν, in a scalar reflectometer measurement, |w|, is the largest difference between |w| and |z|. It is given by the expressions (see Appendix B): WCEν = |C1| + R1 – |w| if R1 > |w| or |C1| > |w|. (17a) If |w| > R1 > |C1| then WCEν = |C1| – R1 + |w|. (17b) which result from the specified values of a", b" c and |w|. The sixth column identifies the appropriate equation, as determined by the relative values of |w|, R1 and |C1|, for computing WCEν. Finally, the computed value of WCEν appears in the seventh column. Notice that WCEν decreases as |w| increases when the phase of c is 0 in cases 1 and 2. On the other hand, WCEν increases with increasing |w| when the phase of c is 180°. The expressions for worst-case error, WCEν, given in Eqs. (17a,b,c) followed quite naturally from the physical operation of a 4-port scalar reflectometer. To conform with current popular practice, one could try to re-cast these expressions into “standard uncertainties” or “expanded uncertainties” as recommended by the U.S. Guide to the Expression of Uncertainty in Measurement (or Guide). [3] It appears, however, that this would not be appropriate. Paragraph 3.2.4 of the Guide states: “It is assumed that the result of a measurement has been corrected for all recognized significant systematic effects and that every effort has been made to identify such effects.” There are three sources of systematic effects in the expression for w from a scalar reflectometer, namely the q”, r” and s parameters in Eq. (15). These are recognized and significant quantities and produce systematic effects that depend on the value of the device measured. Equations (2.153) and (2.154) lead to Fig. 2 which graphically shows the systematic effects of the three error sources. While it is possible to correct for these systematic effects in vector network analyzer systems, it is not possible in scalar systems. The Guide does offer an example (in section F.2.4.5) of a way to incorporate “a single mean correction” into the combined variance to accommodate situations where it is impossible or undesirable to eliminate known systematic effects. It appears, however, that such a procedure would produce an uncertainty statement that is not related to the physical processes in the reflectometer which cause the errors, as shown in Fig. 2. Thus the uncertainty statement is of no added value. 7. C o n clu s io n s If |w| > |C1| > R1 then WCEν = |C1| – R1 – |w|. (17c) Some important remarks about application of Eqs. (17a,b,c) are in order. If one uses the complex values (at a given frequency ν) of a", b" and c to compute the values of R1 and |C1|, then using the appropriate equation of (17) gives WCEν the worst-case error with respect to all possible phases of w at frequency ν. However, WCEν is not worst-case with respect to all possible phases of a", b" and c at frequency ν. It is important to recognize that, at a particular frequency, WCEν may either increase or decrease as |w| increases. This is demonstrated in Table 1 where values of WCEν are computed for a scalar reflectometer having typical values a" = 1, b" = 0.01 and |c| = 0.1. In the first two cases considered, the phase of c is 0°; in the final two cases, the phase of c is 180°. Cases 1 and 3 show the situation when the measured reflection coefficient is |w| = 0.1 while cases 2 and 4 apply when |w| = 0.3. The fourth and fifth columns give calculated (see Eqs. 2.153 and 2.154) values for radius R1 and magnitude of vector C1, respectively, 42 | MEASURE The advantage of using an open/short method for initializing the scalar reflectometer has been quantified; it produces a calibration constant, q", that is very near to unity. On the other hand, the directivity and effective generator reflection coefficient of the reflectometer remain as error sources. It was demonstrated that the reflection coefficient(s) of the detector(s) have no effect on the performance of either 3-port or 4-port reflectometers. It was also shown that the performance of a 4-port scalar reflec- Case c |w| R1 |C1| 1 0.1 0.1 Equation WCEν 0.09991 0.009 (17b) 0.009 2 0.1 0.3 0.29997 0.001 (17b) 0.001 3 –0.1 0.1 0.10011 0.011 (17a) 0.011 4 –0.1 0.3 0.30057 0.019 (17a) 0.020 Ta bl e 1. Dependence of WCEν on |w| and phase of c. www.ncsli.org TECHNICAL PAPERS terms of magnitude |εs| and |ε(r/q)| have been added to the numerator and denominator, respectively, under the radical. Hence, if the difference between the short and open phase angles is (π + ε) radians, the effect is to introduce two secondorder errors, each proportional to ε, into the results obtained for the ideal case when the phase angle difference is exactly π radians. tometer is entirely independent of both the generator level instability (if the detector outputs 3 and 4 are measured simultaneously) and the generator reflection coefficient of its driving generator. A 3-port reflectometer is vulnerable to both. The worst-case error of a scalar reflectometer measurement depends on the scattering parameters of the reflectometer and the value of the reflection coefficient, |w|, of the measured device. Three different expressions for worst-case error from a 4-port reflectometer, depending on the scattering parameters of the reflectometer and |w|, were given. 9. A ppendix B This appendix justifies the expressions for WCE given in Eqs. (17a,b,c), subject to the associated conditions. It also develops the logical conditions for use of the different expressions for WCE. Figure 2 in the body of the report depicted a possible relationship between the loci of w and z. For the purposes of this appendix and convenience, one need consider only the situation when arg(C1) = 0 as shown in Fig. B1. For the sake of even more brevity, all the necessary information in Fig. B1 can be condensed into Fig. B2. The caption of the figure specifies the size order of |C1|, R1 and |w|. In the figure, the points where the loci intersect the real axis are identified as is the tip of the vector C1. The center part of Fig. B2 graphically shows the worst-case error, WCE, as the distance between (1) the point, zr, on the z locus that is most remote from the w locus and (2) the point on the w locus that is nearest to zr. In the right side of the figure, the corresponding R 1 WCE is given. Figures B3 through B7 algebraic expression for show corresponding information for the other five order possibilities. In Figs. B2 through B5, the same expression is used for the C1 WCE while different expressions are required in Figs. B6 and B7. This situation is logically summarized in the Karnaugh map of Fig. B8. Notation used is asz follows: X identifies a “don’t locus 8. A ppendix A The coefficient q", as defined by Eqs. (10) and (11), was derived using the assumption stated in Step 2B of section 3, i.e., that the phase angle of the short used during initialization differed from the phase angle of the open by exactly π radians. Unfortunately, in practice, the two phase angles will nearly always differ by (π + ε) radians, where ε is some small value. Broadband measurements of four different short/open pairs in three different connector types from two different manufacturers revealed a maximum deviation of | ε | = 0.2 radian. The following development examines the effects such an ε deviation. Let the reflection coefficient, z’, of the “offset” short be z' = e–j(π+φ+ε). Equation (9) then takes the form: −qe − j ϕ e − j ε + r ws' = . −se − j ϕ e − j ε +1|w| (9') Divide Eq. (5) by the square root of the product of Eqs. (6) and (9') to obtain: | wm | ws' wo − jϕ +1| | −se − j ϕ e − j ε +1| | q z + r | | se w locus , − jϕ | sz +1| | qe + r | | −qe − j ϕ e − j ε + r | = ( ( ) ) ( ( ) ) or, after manipulation, Eq. (10') which is: | −s 2 e − j 2 ϕ e − j ε + se − j ϕ 1− e − j ε +1| | −s 2 e − j 2 ϕ e − j ε + se − j ϕ 1− e − j ε +1| WCE WCE = |C 1| + R1 - |w| z + r q 2 − j 2ϕ − j ε − jϕ − jε 2 2 − j 2ϕ − j ε − jϕ − jε | −q | −q e e + rqe 1− e + r | e e + rqe 1− e +r2 | | wm | (10') = | sz +1| R ws' wo 1 |w| q | wm | ' ( ) (1− e ) + r | −s 2 e − j 2 ϕ e − j ε + se − j ϕ 1− e − j ε +1| | −q e 2 − j 2ϕ e − jε + rqe − jϕ = − jε 2 | ( ) (1− e ) + r C1 | −s 2 e − j 2 ϕ e − j ε + se − j ϕ 1− e − j ε +1| z +r | −q e 2 − j 2ϕ e − jε + rqe − jϕ − jε 2 | . w locus | sz +1| ws wo zF i g u re B1. A possible C relationshipzbetween w and z loci when r 1 arg(C1) = 0. | −e − j 2 ϕ + jε ( r q ) e − j ϕ + ( r q ) | (10") 2 | wm | ws' wo | −s 2 e − j 2 ϕ + jεse − j ϕ +1| ' ws wo WCE = |C 1| + R1 - |w| WCE wε). If ε << 1, cos(ε) ; 1 and w Observe that e–jε = cos( ε) – j sin( sin(ε) = ε, so Eq. (10') simplifies to Eq. (10"), which is: | −s 2 e − j 2 ϕ + jεse − j ϕ +1| | wm | z locus ≈ | −e − j 2ϕ + jε ( r q ) e − jϕ + (r q ) | 2 ≈ z + (r q ) | −e + jε ( r q ) e − jϕ | sz +1| w | −s 2 e − j 2 ϕ + jεse − j ϕ +1| − j 2ϕ z + (r q ) + (r q ) | 2 | −s 2 e − j 2 ϕ + jεse − j ϕ +1| | −e − j 2 ϕ + jε ( r q ) e − j ϕ + ( r q ) | 2 WCE = |C 1| + R1 - |w| WCE w z C 1 z r . | sz +1| WCE = |C 1| + R1 - |w| WCE Equation (10") is the same as Eq. (10) except that second-order F i g u re B2. Condition: |C1| > R1> |w|. Vol. 1 No. 4 • December 2006 MEASURE | 43 w w z TECHNICAL PAPERS z C1 r WCE = |C 1| + R1 - |w| WCE w w z z C1 r R1 > | C1 | ________________ 2 - B6 WCE = |C 1| + R1 - |w| WCE X wC z w z 1 F i g u re B3. Condition: | C 1| > |w | > r R 1. 1 - B5 X 3 - B7 1 - B4 1 - B2 1 - B3 | C1| > | w| ________________ R 1 > |w| WCE z w wC WCE = |C1 | + R1 - |w| z 1 r WCE WCE = |C1 | + R1 - |w| wC z w z 1 F i g u re B4. Condition: R1 > |C1| >r |w|. WCE z w C1 w WCE = |C1 | + R1 - |w| z F i g u re B8. Karnaugh map of expressions for WCE. care” or impossible condition. The expression for WCE used in Figs. B2 through B5 is identified as expression “1” and is denoted by a leading “1 –“. The expression for WCE used in Fig. B6 is identified as expression “2” and is denoted by a leading “2 –“. The expression for WCE used in Fig. B7 is identified as expression “3” and is denoted by a leading “3 –“. The figure where each expression appears is indicated as a trailing (for example) “– B7”. The map indicates that: • Expression 1 should be used if R1 > |w| or |C1| > |w|. • Expression 2 should be used if |w| > R1 and R1 > |C1|, i.e. if |w| > R1 > |C1|. r • Expression 3 should be used if |w| > |C1| and |C1| > R1, i.e. if |w| > |C1| > R1. WCE = |C1 | + R1 - |w| WCE w z C F i g u re B5. Condition: R1 > |rw| > |C1|. 1 z r WCE w C z 1 w WCE = |C 1| - R1 + |w| w Z r C1 Z F i g u re B6. Condition: |w| > R1 > |C1|. Zr C 1 Z WCE 11. Re f e re n c e s [1] D.M. Kerns and R.W. Beatty, Basic Theory of Waveguide Junctions and Introductory Microwave Network Analysis, Pergamon Press, Oxford, 1967. [2] G.F. Engen, “Amplitude stabilization of a microwave signal source,” IRE Trans. Microwave Theory and Technique, vol. MTT6, pp. 202-206, 1958. [3] “U.S. Guide to the Expression of Uncertainty in Measurement,” ANSI/NCSL Z540-2-1997, National Conference of Standards Laboratories International, Boulder CO. WCE = |C | - R1 - |w| WCE w 10. Ac kn ow le dg e me nt s The measurements made by James A. Woods and cited in Appendix A are gratefully acknowledged. WCE = |C 1| - R1 + |w| WCE w w z 1 w WCE = |C | - R1 - |w| 1 F i g u re B7. Condition: |w| > |C1| > R1. 44 | MEASURE www.ncsli.org Vol. 1 No. 4 • December 2006 MEASURE | 45 TECHNICAL PAPERS A Direct Comparison System for Measuring Radio Frequency Power (100 kHz to 18 GHz) R onal d G i nl ey A b s t r a c t : A direct comparison power measurement system has been developed to measure power sensor effective effi- ciency in the 100 kHz to 18 GHz frequency range. This system is capable of measuring thermistor and thermoelectric based power sensors. Several problems needed to be addressed in the development of the system, including RF leakage from the power sensors and its effect on system electronics, the sensitivity of the power meter and digital volt meter to extraneous signals, and the effect of compensation beads, if there were any, in the sensors. This article covers these problems, provides a discussion of the system design, presents the uncertainty analysis for the system, and finally compares measurement results to measurements made using the NIST 0.05 GHz to 50 GHz system and the voltage/ impedance technique. 1 . I n t ro d u c t i o n A system’s output power level is frequently the critical factor in the design, and ultimately in the purchase and performance, of almost all radio frequency and microwave equipment. [1] Measurements of microwave power are important for a wide array of electronic devices. Equipment, whose characteristics and performance are determined by microwave power measurements, can be found in the areas of communications, aerospace, navigation, surveillance, manufacturing, medical, and consumer electronics. Power measurements are also the foundation for many other microwave measurements such as attenuation and impedance. This paper describes a new system for measuring microwave power at the National Institute of Standards and Technology (NIST). This system covers the frequency range of 100 kHz to 18 GHz and approximately 1 mW to 10 mW. This is the first NIST system that is capable of covering this entire frequency range with one system. The system is also Resistive Power Splitter 100 kHz to 20 GHz Signal Generator 46 | MEASURE Monitor Power Sensor Power Meter DVM Power Meter DVM F i g u re 1. Block Diagram of the 100 kHz to 18 GHz direct comparison system. capable of measuring thermoelectric type of standards across the full 100 kHz to 18 GHz range. This system is similar to the NIST 0.05 to 50 GHz system, but we were previously unable to measure devices below 50 MHz. The basic theory and design of the system will be covered. Some of the problems encountered in the development of the system will be briefly described. The uncertainty components for the system will be briefly discussed. Finally the new system and existing NIST systems will be compared. Ronald Ginley Electromagnetics Division National Institute of Standards and Technology1 325 Broadway Boulder, CO 80305 USA Email: rginley@boulder.nist.gov Standard or DUT 2 . Sy st em De si gn an d Th e ory Figure 1 shows a block diagram of the system. Overall the system is very simple. A signal generator sends an RF signal into a resistive power splitter that then splits the signal between a monitor detector and either the calibration standard detector or the device under test (DUT). The detectors are connected to power meters whose output is connected to a digital volt meter (DVM) in the case of a thermistor type detector, or whose output is read directly by a connected computer through an instrument interface bus for a thermoelectric type of detector. This type of system design is not new and has been used at NIST before. [2, 3] The calibration and measurement processes are detailed in reference. [2] A synopsis of the calibration and measurement process follows. To calibrate the 1 U.S. Government work is not protected by U.S. copyright. www.ncsli.org TECHNICAL PAPERS Ka = Pdc–std η std PM–std M gl–std , (1) Effective Efficiency where: Pdc-std is the power read from the calibration standard; ηstd is the known effective efficiency of the calibration standard; PM-std is the power read from the monitor detector during the calibration; and Mgl-std is the mismatch factor from the reflection coefficients of the standard and the splitter. The process for measuring the ηe of the DUT is the reverse of the calibration process. From the power readings at the monitor detector and the DUT, the 0.995 reflection coefficient of the DUT, the reflection coefficient looking into the test 0.990 port, and Ka, the η e of the DUT can be determined. Note0.985 that all of the power readings are used in ratios (standard or 0.980 DUT to the monitor detector) and are never used as an0.975 absolute power value. By measuring the powers always in ratio, any drift of the signal 0.970 power amplitude is negated. The measurement process is represented by: 0.965 η = DUT where 0.960 Pdc–DUT Ka P 0.955 M–DUT Mgl–DUT (2) 0.950 ηDUT is the effective efficiency of the 0.945 DUT; 0 5 Pdc-DUT is the power read from the DUT; PM-DUT is the power read from the monitor detector during the DUT measurement; and Mgl-DUT is the mismatch factor from the reflection coefficients of the Vol. 1 No. 4 • December 2006 DUT and the splitter. Several reflection coefficients are used during the measurements. This is necessary to correct for the impedance mismatches between various components of the system (Mgl-std and Mgl-DUT). The reflection coefficient of the standard and the DUT are measured directly on vector network analyzers (VNAs), and the reflection coefficient looking into the test port of the splitter is determined with a modified VNA calibration scheme. [4] To summarize the operation of the system, a standard with known effective efficiency is used to determine a value that relates the power at the test port to the power measured in the monitor detector. This value is then used to determine the effective efficiency of an unknown device from the power readings of the unknown device and the monitor detector. The impedance mismatches are corrected for throughout the process. 3 . C a l i b r a t i o n S ta n d a rd s It is not possible to cover the entire working frequency range of this system with one standard. Two standards are used. The first covers the 100 kHz to 50 MHz range. This standard is calibrated using voltage and impedance measurements. From the voltage and the equivalent parallel input resistance, the effective efficiency of the detector can be determined. [5] The 1σ uncertainty in the calibration of the effective efficiency of the standard ranges from 0.00046 to 0.00133 as the frequency goes from 100 kHz to 50 MHz. The second type of standard, used from 50 MHz to 18 GHz, is evaluated in the NIST microcalorimeter. [6] The 1σ uncertainty for this evaluation ranges from 0.0012 to 0.0021 across the frequency range. 4. P ro b l e m s E n c o u n t e re d i n t h e D e v e l op me nt o f the S y s te m 4 .1 C ompe n sat ion Bea ds There are several different types of thermistor based devices that customers send in to NIST for measurement. One of these, which comprise a significant portion of our workload, is manufactured by Hewlett-Packard/Agilent Technologies. [7] These detectors have an extra set of thermistor beads that are used to compensate the HP 432A power meter reading for thermal drift. Because of how the compensation beads are situated in the circuit of the detector, there should not be any interaction between the compensation beads and the measurement beads. [8] However, it has recently been discovered that this is not true. Figure 2 shows the results for measuring an Agilent model 8478B thermistor detector with and without the compensation beads biased. 0.995 0.990 0.985 Effective Efficiency system, a detector with a known effective efficiency (ηe) is connected to the test port of the power splitter. From the known ηe of the standard, the reflection coefficient of the standard, the reflection coefficient looking back into the splitter, and the power measured in both the standard and the monitor detector, a value can be determined (Ka) for each measurement frequency which relates the power available at the test port to the power measured in the monitor detector. With Ka known, the DUT can be measured. Mathematically, the calibration is represented by: Biased Not Biased 0.980 0.975 Biased 0.970 Not Biased 0.965 0.960 0.955 10 15 20 0.950 0.945 0 5 10 15 20 Frequency (GHz) F i g u re 2. Effect of biasing the compensation bead. MEASURE | 47 TECHNICAL PAPERS 1.025 1.02 1.025 1.015 1.02 1.01 PM1 1.015 PM2 Effective Efficiency 1.005 1 0.995 0.99 PM3 1.01 PM4 1.005 1 0.995 0.985 0.1 0.2 0.3 0.4 0.99 0.985 0.1 0.5 0.6 0.7 0.8 0.9 1 Frequency (MHz) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frequency (MHz) F i g u re 3. Measurement results for different power meters. It can be seen in Fig. 2 that a definite shift occurred in the results when the compensation beads were biased compared to when they were not biased. This difference is detector-dependent and can be as large as 0.015 (in effective efficiency), which is much larger than the uncertainty of the measurement (approximately 0.006 to 0.0075). This effect is generally seen in the range from 15 GHz to 18 GHz. We believe that leakage of the RF signal between the sets of beads through some of the blocking capacitors in the detector is causing the effect. Note that at frequencies below 15 GHz there is basically no difference between the measurements. For detectors that have a lower operating frequency range, 1 MHz to 1 GHz, there are no differences caused by the different biasing conditions. 4 .2 E f f e c t o f D e t e c t o r R F L e a k a g e on P o w er M eter s and D VM s Filtering of the RF signals from the dc leads in the detectors at low frequencies is difficult. Not all of the RF energy is absorbed by the thermistor measurement beads and not all of the unabsorbed energy is blocked from the dc leads going to the power meter and on to the DVMs. This energy creates problems. The power meters basically do not react to the extra signal; they may attenuate the signal, but that is about the 48 | MEASURE extent of the interaction. The DVMs will react to the RF leakage signal. The frequency of the signal is important in how it interacts with the DVM. Figure 3 shows the results for measurements using several different power meters. Power Meters 1 and 2 (PM1 and PM2) are of the same design and PM3 and PM4 are of the same design which is different than PM1 and PM2. Several things are happening to the RF signal in these measurements. The power meter is filtering the RF signal that has leaked through the detector, and the DVMs are reacting to the extraneous signal. PM1 and PM2 do not filter the signal as much as PM3 and PM4. It is also apparent that PM1 and PM2 do not filter the signal equally. To support our assertion that it is the RF signal affecting the DVMs, measurements were made with a nominal 100 Hz low pass filter in the leads between the power meter and the DVMs. In these measurements the initial results from power meters like PM1 and PM2 were consistent with the results from PM3 and PM4. Further measurements are underway to verify this effect. 5. Unc er ta inty C om pone nts There are several main uncertainty components for this system. These include the contributions from the calibration standard, the mismatch correction, the system electronics, device variability, and a component from an external power meter, if one is used with a thermoelectric detector. The main components for the 50 MHz to 18 GHz system have been detailed in references [2] and [3]. The expanded uncertainty (coverage factor = 2) runs PM1 from approximately 0.003 to 0.0076 PM2 (50 MHz to 18 GHz) for a therPM3detector, and from approximately mistor 0.013PM4to 0.016 for a thermoelectric sensor. The differences in uncertainty components for this new system occur in the 100 kHz to 50 MHz range. The main differences are from the calibration standard used below 50 MHz (detailed in the calibration standards section above) and from the different VNA that must be used below 50 MHz for reflection coefficient measurements. This VNA has different residual directivity, source match and reflection tracking terms. These are accounted for in the uncertainty of the reflection coefficient measurements, which are propagated into the mismatch uncertainty. [2] For a thermistor sensor, the expanded uncertainty ranges from 0.003 to 0.0045 and for a thermoelectric the uncertainty is approximately 0.0126 to 0.013 (100 KHz to 50 MHz). The uncertainty analysis is still being refined. 6 . M eas ur e m e n t R e s u l t s We have compared measurements from the new system to measurements made by the voltage and impedance technique (Fig. 4) and to measurements made on the NIST 0.05 GHz to 50 GHz Direct Comparison System (Fig. 5). The results in Fig. 4 show fairly good agreement between the two different measurement techniques. The vertical bars are error bars (for three different frequencies) based on the uncertainty of measurements on the new system. It can be seen that the difference in the two results is much less than the uncertainty. We do not yet know what is causing the systematic difference between the different results. More than likely, it is the result of slight errors in the values for the calibration standards. The results from the new system are showing very good agreement with the existing 0.05 GHz to 50 GHz system. The error bars in Fig. 5 show the uncerwww.ncsli.org TECHNICAL PAPERS tainty for the measurements from the new system at a few selected points. There are slight deviations at the higher frequencies, which are in the normal range of differences for measurements at these frequencies and are1.006 much less than the uncertainty of the measurement. 1.006 1.004 Effective Efficiency 1.002 1.004 7. Conclus ion 1.002 Effective Efficiency A new system for measuring microwave 1.000 power (effective efficiency) has been 0.998 developed at NIST. This is the first NIST system able to cover the entire frequency 0.996 range of 100 kHz to 18 GHz. Also, this 0.994 is the first NIST system able to measure thermoelectric detectors 0.992 below 50 MHz. Several factors became important in the 0.990 design of the system. These included the RF leakage through the detectors and the 0.988 subsequent effects on the power meters 0.986 and DVMs, and the effect of biasing the 0.984 compensation bead in thermistor detec0.1 tors that have one. The uncertainty of the system either uses the existing analysis from the NIST 0.05 to 50 GHz system at the higher frequencies or includes terms for the different calibration standard and reflection coefficient determination for the lower frequencies. The new uncertainties are comparable to the existing ones. Finally, measurements on the new system agree very well with measurements from the existing direct comparison system and from the voltage and impedance technique. Effective Efficiency 0.99 0.98 0.97 0.96 0.998 0.996 Voltage/Impedance New System 0.994 0.992 Voltage/Impedance 0.990 New System 0.988 0.986 0.984 0.1 1 10 100 Frequency (MHz) F i g u re 4. Comparison of measurements on the new system to the voltage/impedance 1 10 100 technique. Frequency (MHz) 1 0.99 Effective Efficiency 1 1.000 0.98 0.97 0.96 8 . Re f e re n c e s 0.95 0.94 0 [1] Hewlett-Packard, “Fundamentals of RF and microwave power measurements,” Hewlett-Packard Application Note 12 64-1, 2 4 6 8 10 August 1977. [Note: This Application Note has been updated by Agilent Technologies as four separate documents (AN 1449-1/2/3/4) available from the web site: agilent.com] [2] M.P. Weidman, “Direct Comparison Transfer of Microwave Power Sensor Calibrations,” NIST Technical Note 1379, January 1996. [3] J.R. Juroshek, “NIST 0.05-50 GHz Direct-Comparison Power Calibration System,” Proceedings IEEE CPEM 2000, May 2000. [4] J.R. Juroshek, “A Direct Calibration Method for Measuring Equivalent Source Mismatch,” Microwave Journal, pp. 106-118, October 1997. Vol. 1 No. 4 • December 2006 Old System New System 0.95 Old System 14 16 18 20 0 2 New System 0.94 4 6 8 10 12 14 16 18 20 Frequency (GHz) F i g u re 5. Comparison of results from the new 100kHz to 18 GHz system to those from the old 0.05 GHz to 50 GHz system. [5] A.Y. Rumfelt and L.B. Elwell, “Radio frequency power measurements,” Proc. IEEE, vol. 55, (6), pp. 837-850, June 1967. [6] F.R. Clague, “Coaxial Reference Standard for Microwave Power,” NIST Tech. Note 1357, April 1993. [7] Products or companies named here are cited only in the interest of complete scientific description, and neither consti- tute nor imply endorsement by NIST or by the US government. Other products may be found to serve just as well. [8] Technical Manuals for the HP Model 8478B and 478A Thermistor Detectors. [Available from Agilent Technologies web site under Technical Support, Manuals and Guides: home.agilent.com/ agilent/home.jspx?cc=US&lc=eng] MEASURE | 49 TECHNICAL PAPERS Remote Time Calibrations via the NIST Time Measurement and Analysis Service M i c h a e l A . L o m b a rdi a nd A n dre w N. N o vi c k A b s t r a c t : The National Institute of Standards and Technology (NIST) now offers a new remote calibration service designed to assist laboratories that maintain an accurate local time standard. The service monitors the local time standard by continuously comparing it to the national time standard and reports the comparison results to the customer in near real-time. This new service, called the NIST Time Measurement and Analysis Service, or TMAS, works by making simultaneous common-view measurements at NIST and at the customer’s laboratory with up to eight Global Positioning System (GPS) satellites. Each customer receives a time measurement system that performs the measurements and sends the results to NIST via the Internet for instant processing. Customers can then view their standard’s performance with respect to NIST in near real-time, using an ordinary web browser. Time is measured with a combined standard uncertainty of less than 15 nanoseconds, and frequency is measured with an uncertainty of less than 1 #10–13 after 1 day of averaging. This paper describes the multi–channel GPS common–view technique used by the service and the measurement system sent to each customer. It also explains how NIST calibrates each measurement system prior to shipment, how measurement results are reported to the customer, and how the measurement uncertainties are estimated. 1. I nt ro d u c t i o n There is a small but growing demand for calibration laboratories and research facilities to maintain a high accuracy time standard. This requires the laboratory to continuously generate a 1 pulse Michael A. Lombardi and Andrew N. Novick Time and Frequency Division National Institute of Standards and Technology1 Boulder, CO 80305 USA Email: lombardi@nist.gov 50 | MEASURE per second (pps) on-time signal, and for laboratories in the United States, to be able to state the uncertainty of that signal with respect to the Coordinated Universal Time (UTC) scale maintained at the National Institute of Standards and Technology (NIST), known as UTC(NIST). Once the uncertainty of the 1 pps signal is known, it can then be used as a standard for traceable measurements of time interval and/or frequency, or as a synchronization source for other timing systems. High accuracy 1 pps signals are normally generated by either a cesium oscillator or a Global Positioning System disciplined oscillator (GPSDO). Cesium oscillators are primary laboratory standards that physically realize the base unit of time interval (the second) as defined by the International System (SI). How- 1 This paper is a contribution of the United States government, and is not subject to copyright. An earlier version of this paper appeared in the 2006 NCSLI Conference proceedings. The identification of commercial equipment is for purposes of illustration only, and does not imply endorsement by NIST or by NCSL International. www.ncsli.org TECHNICAL PAPERS ever, they still need to be synchronized before serving as a time standard. GPSDOs are devices that usually contain a quartz or rubidium oscillator whose outputs are continuously steered to agree with signals from the Global Positioning System (GPS) satellites. In contrast to a cesium oscillator, a GPSDO is inherently on-time, and can produce a 1 pps signal that is usually well within 1 µs of UTC. However, because it is not usually possible to measure the time offset of a GPSDO with respect to UTC(NIST), laboratories are often limited to using and trusting the number quoted on the manufacturer’s specification sheet as an uncertainty figure. Laboratories that want their time standards calibrated against UTC(NIST) to accuracies better than 1 µs have historically had several options, all of which have some shortcomings. Customers sometimes ask to send their cesium oscillator to NIST for calibration, but this is normally not a good solution, nor is it practical. NIST offers several frequency calibration services for cesium oscillators that are sent to Boulder (Service IDs 77100C, 77110C, and 77120C) [1], but time information is lost during the shipment to NIST and the return shipment to the customer, and the cesium would need to be resynchronized when it returns to the customer’s lab. In fact, when the device returns to the customer, even the frequency of the device might be substantially different from what it was during the calibration. A GPSDO can be sent to NIST for delay calibrations (Service ID 76120S). [1] This works well if the antenna and cable are calibrated along with the receiver. However, due to local reception conditions, the device might perform differently at the customer’s site than it did at NIST, and the customer will be without a time reference during the interval when the unit is gone from their laboratory. The NIST services described in the above paragraph follow the traditional model, common in most fields of metrology, where the device under test (DUT) is sent to another laboratory for calibration. In these cases, the DUT is sent to NIST, where it is calibrated and then returned to the customer along with a report containing the measurement Vol. 1 No. 4 • December 2006 results and an uncertainty statement. This calibration is typically repeated at an interval determined by the customer, for example, once every year. The field of time and frequency typically uses a different model, based upon remote calibration. Unlike the traditional model, a remote calibration does not require the customer to send their DUT to NIST. Instead, the DUT remains in place at the customer’s site, and NIST sends a measurement system to the customer. The measurement system then collects data that are sent back to NIST for processing, and the calibration can last for as long as the customer wants it to last. Laboratories that want their standard to be continuously monitored by NIST can do so by subscribing to a remote calibration service and have their standard continuously compared to UTC(NIST) every day of the year. NIST has offered remote frequency and time calibration services since 1983. [2] The original remote time calibration service, called the Global Time Service (GTS), was launched that year and continues to serve a number of customers. However, its technology is now outdated in some respects. For example, there are gaps in the measurement data because the satellites are not continuously tracked. Instead, satellite data are recorded during a series of scheduled tracks that last for only 13 minutes each, and the single-channel receivers supplied to some GTS customers track just one satellite at a time. Perhaps more importantly, the GTS does not allow customers a convenient way to view their measurement results until they receive their monthly reports in the mail. With today’s technology, it seems the ultimate solution to a customer’s time measurement problem would be to have their standard compared to UTC(NIST), 24 hours a day, 7 days a week, with the results continuously updated via the Internet so that they can easily be accessed from anywhere. This is the solution provided by the new NIST Time Measurement and Analysis Service (TMAS), the subject of this paper. The TMAS offers measurement uncertainties that are essentially equivalent to the GTS, but it costs significantly less, and has the advantage of making its measurement results available to customers in near real-time via the Internet. 2. P h y s i c a l D e s c r i p t i o n o f t h e T M A S M e a s u re m e n t S y s t e m The TMAS was announced in late 2005 and assigned a Service ID of 76101S by the NIST calibration office. [1] The service shares hardware technology previously developed for the NIST Frequency Measurement and Analysis Service (FMAS) [3], and software technology previously developed for the Interamerican Metrology System (SIM) time and frequency comparison network. Thus, the same technology delivered to TMAS customers has been proven by continuously comparing the national time scales of the National Research Council in Canada, UTC(NRC), and the Centro Nacional de Metrologia (CENAM) in Mexico, UTC(CNM), to each other and to UTC(NIST), with excellent results. [4] Customers who subscribe to the TMAS receive a measurement system consisting of an industrial rack-mount computer, an LCD monitor, and a keyboard with an integrated trackball (Fig. 1). A time interval counter with a single shot resolution of about 30 ps and an eight-channel GPS receiver are embedded inside the computer case. [3] The system is assembled by NIST prior to shipment and is easy to install. The customer is required only to connect four cables to the back panel of the system, as listed in Table 1. When signals are connected and the unit is powered on, it will begin taking measurements and sending data back to NIST. F i g u re 1. The TMAS measurement system. MEASURE | 51 TECHNICAL PAPERS 3. T h e C o m m o n- Vi e w M e a s u re m e n t Te c h n i q u e Connector Input Signal Type Description Counter Time Base BNC The time interval counter requires either a 5 or 10 MHz sine wave signal as its external time base. This can often be obtained from the same DUT that provides the time standard. This connection is made with coaxial cable (typically RG-58). Time Standard BNC The customer’s 1 pps time standard is connected to the measurement unit using a coaxial cable (typically RG58). The delay of this cable must be measured by the customer and entered into the system software. GPS Antenna TNC The GPS antenna and cable are included with the system and calibrated at NIST prior to shipment, and a delay value is already entered into the system (Section 4). The length of the antenna cable is specified by the customer before the calibration is started. After the system arrives at the customer’s site, the customer is responsible for mounting the antenna on a rooftop location with a clear view of the sky on all sides. The antenna is small and easy to mount. Network Ethernet An Ethernet interface is used to connect the system to the Internet. The customer is required to provide an alwayson Internet connection with a dedicated IP address. The system transmits measurement data using the file transfer protocol (FTP), and ports 20 and 21 must be left open if the system resides behind a firewall. Ta bl e 1. TMAS input signals. 52 | MEASURE The TMAS employs the common-view measurement technique to compare time standards located at remote locations from each other. Ideally, a comparison between two time standards would be made by bringing them into the same laboratory and connecting them both to some type of phase comparator, usually a time interval counter. If bringing the time standards together into the same lab is not practical or desirable, the difference between the two time standards can still be measured by simultaneously comparing both standards to a common reference signal that can be received at both sites. Both sites record their measurements and exchange their results, and the results are subtracted from each other to obtain the time difference between the two standards. The common-view signal can be thought of as a transfer standard and its value drops out of the final measurement result. To visualize how the common-view technique works, imagine two people living at opposite ends of a small town who want to compare the time displayed by the grandfather clocks in their living rooms. This would be an easy problem to solve if they could get the clocks together in the same place and compare them side by side. However, moving the clocks would be difficult and is not practical or desirable. Therefore, each person agrees to write down the time displayed by their clock when a fire whistle (located midway between them) blows in their town, an event that happens periodically. After writing down the readings, they call or email each other and exchange the time readings. If the first clock read 12:01:35 and the second clock read 12:01:47, then simple subtraction tells them that the second clock was 12 seconds ahead of the first clock when the fire whistle blew. The time when the fire whistle blew is unimportant. It only matters that it was heard at the same time, and that a simultaneous measurement was made at both houses. If so, the measurement reveals the time difference between the two grandfather clocks and the comparison was successful. [5] The common-view technique has been used in the time measurement world for many decades, with a number of different types of signals used as transfer standards. One notable common-view measurement involved radio station WWV. From 1955 to 1958, the United States Naval Observatory (USNO) in Washington, D.C. and the National Physical Laboratory (NPL) in Teddington, United Kingdom made simultaneous commonview measurements of the signals broadcast from WWV, which was then located in the Washington, D.C. area. The USNO compared WWV to an astronomical time scale (UT2), and NPL compared WWV to the new cesium standard they had just developed. The resulting measurement helped the USNO and NPL equate the length of the astronomical second to the atomic second, eventually leading to the atomic second being defined as the duration of 9,192,631,770 energy transitions of the cesium atom. [6] In later years, common-view measurements were made with a variety of signals serving as transfer standards, including LORAN-C and television broadcasts, 60 Hz power line signals, and even pulses from optical pulsars. [7] Major advances in accurate common-view measurements began after the first GPS satellite was launched in 1978. Signals from the GPS satellites were a nearly ideal common-view referwww.ncsli.org TECHNICAL PAPERS ence because there was a clear path between the transmitter and receiver, and because the lengths of the two paths between the transmitter and receivers were nearly equal. Common-view GPS measurements began at NIST (then known as NBS) shortly after the first GPS satellite was launched [8], and as previously mentioned, a common-view service was in place by 1983. [2] The performance of common-view GPS measurements was some 20 to 30 times better than results previously obtained using LORAN-C as a transfer standard [9], and the common-view GPS technique soon played a central role in the international calculation of UTC performed by the International Bureau of Weights and Measures (BIPM), as it does to this day. [10] Common-view GPS comparisons use one or more GPS satellites as the common-view reference (Fig. 2). There are several variations of the technique, but all have the same objective, to compare time or frequency standards located at remote locations. The common-view method involves a GPS satellite (S), and two receiving sites (A and B), each containing a GPS receiver, a time interval counter, and a local time standard. The satellite transmits a time signal that is nearly simultaneously received at A and B, and a measurement is made at both A and B that compares the received GPS signal to the local time standard. Thus, the measurement at site A compares the GPS signal received over the path dSA to the local clock, Clock A – S. Site B receives GPS over the path dSB and measures Clock B – S. The two receivers then exchange and difference the data. Delays that are common to both paths dSA and dSB cancel out, but delays that aren’t common to both paths contribute uncertainty to the measurement. The result of the measurement is (Clock A – Clock B) with an error term of (dSA – dSB). Thus, the basic equation for common-view GPS measurements is: (Clock A – S) – (Clock B – S) = (Clock A – Clock B) + (dSA – dSB) (1) The components that make up the (dSA – dSB) error term can be measured or estimated (Section 8) and applied as a Vol. 1 No. 4 • December 2006 F i g u re 2. Common-view GPS. correction to the measurement and/or be accounted for in the uncertainty analysis. The (dSA – dSB) error term includes not only delays from the satellite to the receiving antennas, but also delays that take place after the signal is received. Therefore, a key to a successful measurement is to have well understood and characterized delays at each site. This means that the common-view systems must be calibrated so that their relative delays are as close to zero as possible. The calibration of TMAS units is done at NIST prior to shipment to the customer, and is discussed in Section 4. 3 .1 C o mm o n- Vi e w an d Tr a c e a b i l i t y For obvious reasons, the common-view technique simplifies a laboratory’s task of establishing traceability to the SI. Calibration laboratories are generally required to establish traceability of their own measurement standards and measuring instruments to the SI by means of an unbroken chain of calibrations or comparisons. The link back to the SI is normally achieved through measurements that can be traced to the measure- ment standards maintained by a national metrology institute (NMI), the role filled by NIST in the United States. Therefore, laboratories can establish traceability to the SI by sending their standard to NIST for calibration, or to another laboratory that has had its standard calibrated by NIST (which of course introduces another “link” in the traceability chain). Even then, however, traceability is established only at a given point in time, and needs to be periodically reestablished. [11] For example, if a standard had been calibrated by NIST ten years ago, a laboratory auditor or assessor would probably not consider that to be sufficient evidence to establish traceability today. The TMAS completely solves the traceability problem. If we equate the TMAS to the model described in Section 3 above, Clock A is the time standard maintained at the customer’s site, and Clock B is the national time standard maintained by NIST. Thus, the TMAS makes it possible to continuously establish traceability by making continuous, direct comparisons against the national standard. This means that the traceability chain back to the NMI contains only one link [12], which is the optimal situation for obtaining the best measurement results. 4. C a l i b r a t i o n o f M e a s u re m e n t S y s te ms Pr i or t o Sh ip me nt f ro m N I S T Each measurement system is calibrated at the NIST Boulder laboratories prior to being shipped to the customer. The calibration is done by the common-clock method, where the system under test and the reference system at NIST are both measuring the same clock, a 1 pps signal from the UTC(NIST) time scale (Fig. 3). The customer’s system is installed at NIST using the same antenna and cable that will be shipped to the customer. The antenna is attached to a previously surveyed mounting pole whose coordinates are known to within an uncertainty of less than 20 cm. The length of the baseline between the customer’s antenna and the reference antenna at NIST is about 6 m. The calibration lasts for 10 days and results in an average delay number, DRx, that is entered into the TMAS system prior to shipment to the customer. MEASURE | 53 TECHNICAL PAPERS 6m GPS Antenna GPS Antenna GPS Receiver GPS Receiver 1 pps 1 pps Time Interval Counter Start Time Interval Counter Stop Start Stop UTC(NIST) F i g u re 3. A common-view common-clock calibration of a TMAS measurement system. The time deviation σx(τ) [13], of the common-clock calibrations is typically 0.2 ns or less at an averaging period of 1 day. Figure 4 shows results for a calibration, where the peak-to-peak variation of the 10 minute averages was less than 10 ns, the average delay DRx was equal to 41.1 ns, and σx(τ) was equal to 0.16 ns at an averaging time of 1 day. There are some outliers in the data, but there appears to be no significant slope or trend. However, the results of a common-view, common-clock calibra- tion will vary slightly when repeated multiple times, introducing a systematic error that must be accounted for in the uncertainty analysis. This will be discussed further in Section 8. 5. Te c h n i c a l D e t a i l s o f t h e T M A S S o f t w a re a n d H a r d w a re The GPS receiver used by the TMAS simultaneously tracks up to eight GPS satellites and outputs a 1 pps signal that is compared to customer’s time standard with a time interval counter. The receiver 46 Common-view, common-clock delay calibration of TMAS unit 45 44 43 Nanoseconds 42 41 40 also provides data used to produce a time offset reading for each individual satellite, and these readings are displayed on the system monitor (Fig. 5). Data are stored in a file containing a header with the current system settings, and GPS data contained in a 32 # 144 matrix. The 32 columns represent the GPS satellites, with each satellite’s data stored in the column whose number equals its pseudo-random noise (PRN) code. The 144 rows represent the number of 10 minute periods in 1 day. At the end of each 10 minute period, the averaged data are sent via the file transfer protocol (FTP) to a NIST web server, where they are reduced and displayed on-the-fly (Section 6) when requested by a customer. As many as 11 520 minutes of data (144 segments # 10 minute tracks # 8 satellites) can be collected per day, with no dead time or gaps between measurements. This exceeds the maximum amount of data collectable by the GTS with a single-channel receiver by a factor of about 18. Note that the software installed on the customer’s measurement system only collects data and sends it to NIST; it does not perform the common-view data reduction. This is done by web-based analysis software developed at NIST as a group of common gateway interface (CGI) applications written with a combination of a compiled BASIC scripting language and a Java graphics library. The software can process up to 200 days of data (28 800 10-minute segments) and display them on one graph. It quickly aligns the common-view tracks where both NIST and customer viewed the same satellite at the same time and performs the common-view subtraction for each aligned track. A time difference, TD, for a single 10 min track is computed as 8 39 TD = 38 37 53814 53813 53812 53811 53810 53809 53808 53807 53806 53805 53804 36 (SatA i − SatBi ) ∑ i =1 , (2) CV where SatAi is the series of individual satellite tracks recorded at site A, SatBi is the series of tracks recorded at site B, and CV is the number of satellite tracks common to both sites. Modified Julian Dates (03/10/2006 to 03/19/2006) F i g u re 4. Results of a 10-day TMAS measurement system calibration. 54 | MEASURE www.ncsli.org TECHNICAL PAPERS 6. R e p o r t i n g R e s u l t s t o the Cust omer F i g u re 5. The TMAS measurement system displays the collected GPS readings. Because all of the data collected by TMAS customers are uploaded to a NIST server, customers can request and view the data whenever they wish. Requests are normally processed within a fraction of a second and can be made using any Java-enabled web browser from any Internet connection, through a password protected web site. The data are graphed as either 1 hour (Fig. 6) or 1 day averages, and the web-based software computes both the time deviation, σx(τ), and Allan deviation, σy(τ) [13], of the entire data set. In addition, 10 minute, 1 hour, or 1 day averages can be copied from the web browser and pasted into a spreadsheet or other application if the customer wants to perform further analysis. At the laboratory’s request, NIST can also provide signed paper copies of TMAS reports. These reports are issued monthly, but contain essentially the same information that is available on-line. The TMAS is a near real-time common-view system, which is a tremendous benefit to the customer. During normal operation, the data will be updated every 10 minutes, meaning that customers can view their time difference with respect to UTC(NIST) within minutes after the measurement was made. Near real-time common-view systems have been implemented previously in Asia [14] and in the SIM region [4], but they are still the exception rather than the rule. Some common-view services do not report results to the customer for days or weeks after the measurements were made. 7. Fi el d Te s t s F i g u re 6. Viewing TMAS data with a web browser. Vol. 1 No. 4 • December 2006 Figure 7 shows the results of a six month comparison (October 2005 to March 2006) between the Sandia National Laboratories' primary time standard and the UTC(NIST) time scale. The Sandia standard is a cesium oscillator located in Albuquerque, New Mexico, a distance of about 561 km from the NIST laboratories in Boulder, Colorado. The red line shows the actual measurement data, and the blue line is a linear least squares fit. The slope of the least squares line is about 1.7 ns per day. This indicates that MEASURE | 55 TECHNICAL PAPERS 850 Sandia Labs Primary Time Standard – UTC(NIST) Sandia Labs Primary Time Standard – UTC(NIST) Uncertainty Component 800 Nanoseconds 750 700 Measurement Data 4 GPS antenna coordinates error 3 Equipment delay changes dueLine to Linear Least Squares environmental factors 3 53830 53820 53810 Modified Julian Dates (10/01/2005 to 03/31/2006) Modified Julian Dates (10/01/2005 to 03/31/2006) 53800 53790 53780 53760 53770 53740 53750 53720 53820 53730 53830 53710 53810 53700 53800 53690 53790 53670 53770 53680 53780 53660 53760 500 Measurement Data Linear Least Squares Line 53650 53750 53730 53640 53740 53720 53710 F i g u re 7. TMAS comparison (six months) between the time standard at Sandia and UTC(NIST). 10 Propagation delay changes due to multipath 2 Errors in modeled ionospheric corrections 2 Error in cable delay measurements made at customer’s site 1 Resolution of instrumentation 0.05 0 Nanoseconds -10 -20 -30 -40 53794 53784 53779 53774 53769 53764 53759 53754 53749 53744 -60 Ta bl e 2. TMAS estimated Type B uncertainties. UTC(NRC) – UTC(NIST) -50 53789 53700 53690 550 53680 Calibration of TMAS measurement unit at NIST 650 600 53670 Uncertainty (nanoseconds) Modified Julian Dates (01/14/2006 to 02/23/2006) age area of this estimated uncertainty, typically within 5 ns, which helps to validate the TMAS performance. F i g u re 8. Comparison between UTC(NRC) and UTC(NIST). 8. TMA S Unc erta inty Ana ly si s the Sandia standard has a mean frequency offset of 1.9#10–14 with respect to UTC(NIST). As described earlier, the TMAS technology has also been field tested by comparing UTC(NIST) to the time scales of other NMIs in the SIM region. [4] Figure 8 shows the result of a 41 day comparison between UTC(NIST) and UTC(NRC), the Canadian national standard, over the 2471 km baseline between Boulder and Ottawa, Canada. NIST and NRC each contribute data to the BIPM that are used to help derive the international UTC time scale. The BIPM publishes these data monthly in their 56 | MEASURE Circular-T document. [15] Figure 8 shows the results of the daily comparisons made with the TMAS technology in blue, and the “official” numbers from the BIPM Circular-T reported at five-day intervals in red. The Circular-T values are obtained with common-view GPS, but are made by different receivers and with the benefit of some extensive post processing, with results reported anywhere from two to eight weeks after the measurements are made. The blue values have error bars reflecting the estimated 15 ns uncertainty of the TMAS (analysis is provided in the next section). The Circular-T values are well within the cover- Estimating the uncertainty of the TMAS involves evaluating both the Type A and Type B uncertainties as described in the ISO standard. [16] Brief examples are given here for both time and frequency. 8 .1 An a l ysis of Ti me U n c er t a in t y To evaluate the Type A time uncertainty, we use the time deviation σx(τ), at an averaging time of 1 day. The time deviation is an industry standard statistic [13] that is calculated automatically by our web-based software. Using the data displayed in Fig. 7, we obtain a Type A uncertainty of 1.2 ns between NIST and Sandia, over a baseline of 561 km. This www.ncsli.org TECHNICAL PAPERS 4 10-day common-clock calibrations of a TMAS system Nanoseconds 3 2 1 0 53830 53820 53810 53800 53790 53780 53770 53760 53750 53740 53730 53720 53710 53700 53690 53680 53670 53660 53650 53640 53630 -1 Modified Julian Dates (September 2005 - March 2006) F i g u re 9. Results of consecutive 10-day common-clock calibrations made over a 190 day interval. uncertainty will increase over longer baselines, but is typically about 1.5 ns for the 2471 km baseline between NIST and NRC. As a result, we expect the Type A time uncertainty to be less than 2 ns for all TMAS customers in the continental United States. The Type B evaluation is more difficult, but we have identified seven components that can potentially introduce systematic errors that are summarized in Table 2 and discussed in more detail in Sections 8.1.1 through 8.1.7. Some Type B uncertainties can also get larger as a function of the length of the baseline, but the estimates provided here should be applicable for all TMAS customers in the continental United States, where the baseline length should not exceed 3000 km. Due to the nature of common-view measurements, any systematic error that is common to both sites will cancel out, so all the Type B components listed here relate to uncertainties that can affect one site differently from the other. All type B uncertainties are treated as normal distributions. In the case of antenna coordinates, we assume that the customer will be able to survey their antenna’s position to within an uncertainty of 1 m. If this is not true, the combined time uncertainty of the TMAS will increase, as explained in Section 8.1.2. Vol. 1 No. 4 • December 2006 8 .1 .1 C a l i b r a t i o n o f T M A S M e a s u r em e n t U n it a t NI S T As described in Section 4, the 10-day common-clock calibrations of TMAS units are typically stable to 0.2 ns or less, but the results are not necessarily repeatable at different times of the year. For example, if a common-clock calibration were continuously repeated, the resulting estimate of DRx would vary by at least several nanoseconds, depending upon which 10-day segment was chosen. [17] This is illustrated in Fig. 9, which shows the results of a unit that was continuously calibrated at NIST over a 190-day interval spanning from September 2005 to March 2006, producing 181 overlapping 10-day segments. During this interval, the peak-to-peak variation is nearly 4 ns, and a unit could be shipped with a DRx value from anywhere within this range. Based on data collected from repeated calibrations of several units, we assign a Type B uncertainty of 4 ns to our delay calibrations. 8. 1 .2 G P S A n t e n na C o o rd i n a t e s E r ro r The customer is required to obtain coordinates for the GPS antenna prior to starting the TMAS measurements. If the customer has a way to independently survey the antenna, the resulting coordinates can be typed in to the TMAS software. If not, the TMAS system can survey the antenna position by averaging position fixes for 24 hours, a method that does an excellent job of determining the antenna’s horizontal position (latitude and longitude) to within less than 1 m. However, GPS does a comparatively poor job of surveying vertical position (elevation), and the vertical position error is usually at least several times larger than the horizontal position error. This is because GPS provides earth-centered coordinates and measures the distance between the center of the earth and the satellite. Vertical position is obtained with the radius of a model of the earth’s surface. There is nearly always some bias in the estimated vertical position due to local terrain that differs from the model. We assign a Type B uncertainty of 3 ns to the GPS antenna coordinates, which assumes that the customer survey is within 1 m (the approximate distance that light travels in 3 ns). However, if the TMAS self survey is used, this uncertainly will probably be larger, as large as 3 ns per meter for some satellites, but closer to 2 ns per meter of position error, on average. Figure 10 shows the result of 20 TMAS antenna surveys conducted at NIST in Boulder, Colorado, each lasting for 24 hours. Each survey was done with the same receiver and an antenna that had been independently surveyed to an estimated uncertainty of less than 0.2 m. The blue line in the figure shows the total position error in the X, Y, Z coordinates based on the distance from the known coordinates, and the red line shows the error in the vertical position for each of the 20 surveys. As shown in Fig. 10, the average position error was 5.37 m, with nearly all of this error due to error in the vertical position, which was 5.30 m. The estimated vertical positions were biased about 4 to 6 m above the actual elevation, resulting in a Type B uncertainty due to antenna coordinates error that would typically exceed 10 ns, much larger than our 3 ns allowance. This might be an acceptable uncertainty for many customers, but for the best results, TMAS customers should have their antenna elevation independently surveyed to within an uncertainty of 1 m. MEASURE | 57 6.5 Position Error (meters) 6 5.5 5 Total Position Error or in multiple 24-hour surveys of 4.5known GPS antenna coordinates 5 6 7 8 9 10 11 12 4 13 14 15 16 17 18 19 Vertical Position Error 20 21 Antenna surveys (one per day)Position error in multiple 24-hour surveys of known GPS antenna coordinates 3.5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Antenna surveys (one per day) F i g u r e 10. Position errors (with respect to known coordinates) from 20 TMAS antenna surveys. 8 .1 .3 T M A S E q u i p m e n t D e l a y C h a n g es D u e t o E n v i ro n m e n t a l F a c t o r s GPS receiver, antenna, and antenna cable delays can change over the course of time due to temperature and other environmental factors. The GPS receiver delay has the largest sensitivity to temperature, but its performance will be very stable if the laboratory temperature is well controlled. The receiver temperature is typically just a few degrees Celsius higher than the laboratory temperature, with a similar range. However, a sudden change in laboratory temperature can sometimes cause the receiver delay to change by several nanoseconds, usually returning to its previous delay when the temperature returns to normal. Smaller receiver delay changes can occur slowly over time for reasons that are not completely understood. These delay changes might be caused by fluctuations in power supply voltages, vibration, or humidity. As a result, we assign a Type B uncertainty of 3 ns to account for receiver/ antenna delay changes due to the environment. The GPS antenna and part of the cable are outdoors, and are thus subjected to large annual variations in temperature (the annual temperature range can exceed 60 °C in Boulder, Colorado). Even with this large of a range, the actual changes in the electrical delay of the cable due to temperature are insignifi58 | MEASURE cant, but can potentially cause the receiver tracking point to change, introducing phase steps in the data. [18] The TMAS guards against this possibility by using a high quality antenna cable with a low temperature coefficient. 8 .1 .4 P r o p a g a t i o n D e l a y C h a n g e s Due to Multipath Errors due to multipath are caused by GPS signals being reflected from surfaces near the antenna. These reflected signals can then either interfere with, or be mistaken for, the signals that follow a straight line path from the satellite. TMAS customers are instructed to mount their antennas in an area with a clear, unobstructed view of the sky on all sides, and an antenna is used that was designed to reject multipath signals. For these reasons, the uncertainty due to multipath is usually very small. However, because some errors due to multipath are difficult to detect and avoid, we assign a Type B uncertainty of 2 ns. [19] 8 .1 .5 P r o p a g a t i o n D e l a y C h a n g e s D ue t o I on os pher i c Co ndi ti o ns The GPS signals are line of sight, and the path delay between the satellites and the receiver can be accurately estimated from the distance and the speed of light. However, the signals are bent slightly as they pass through the ionosphere and troposphere, changing their propagation delay. The delay changes are largest for satellites at low elevation angles. The GPS satellites broadcast a modeled ionospheric delay correction that is automatically applied by the TMAS to the measurements made at both sites. However, ionospheric conditions are not identical at both sites (particularly when it is dark at one site and daylight at the other), and some common-view GPS systems apply ionospheric corrections as measured at each site, instead of using the broadcast corrections. [19] This delays the processing of the measurement by at least one day, but Total Positionresults Error Vertical Position Error reduces the measurement uncertainty. Because the TMAS uses modeled ionospheric corrections as opposed to measured corrections, we assign a Type B uncertainty of 2 ns for ionospheric delay that should cover all customers in the continental United States. 8 .1 .6 C a b l e D e l a y M e a s u r e m e n t s M a d e a t C u s t o m e r ’s L o c a t i o n When the TMAS unit is installed, the customer is responsible for measuring the reference delay, or DREF, and entering this value into the system software. The reference delay represents the delay from the local time standard to the end of the cable that connects to the TMAS system. This is typically a one-time measurement made by the customer with a time interval counter. The Type B uncertainty will normally not exceed 1 ns if proper measurement techniques are followed. 8 .1 .7 R e s o l u t i o n U n c e r t a i n t y o f S o f t w are an d I ns tr u me nt at i on The TMAS software limits the resolution of the entered delay values to 0.1 ns, contributing an insignificant resolution uncertainty of 0.05 ns. 8 . 1 . 8 C o m b i n ed T i m e U n ce r t ai n ty The combined Type B uncertainty, Ub, is obtained by taking the square root of the sum of the squares of the estimated uncertainties listed in Table 2, and equals 6.6 ns. The combined expanded uncertainty Uc is obtained by this equation: Uc = k Ua2 + Ub2 . (3) If we use a coverage factor of k = 2 and Ua = 2 ns (as discussed in Section 8.1), www.ncsli.org TECHNICAL PAPERS then Uc is equal to 13.7 ns. This figure has been rounded up to a conservative service specification of 15 ns that should be achievable with all customers. In the case of the 2471 km baseline between NIST and NRC, these results have been validated with independent measurements published by the BIPM [15] that fall well within the TMAS coverage area (Fig. 8). 8 .2 An alysis of Fre q u e n c y U n c e r t a i n t y Frequency uncertainty can be estimated by fitting a least squares linear line to the data to obtain a mean frequency offset, Y, and then using 2σy(τ) [13] as the Type A uncertainty Ua (k = 2 coverage). Since there is no significant Type B component for frequency, the combined uncertainty Uc can be considered as the Type A uncertainty. The upper and lower bounds of the coverage area are represented by Y + Uc and Y – Uc, respectively. For the 6month data run shown in Fig. 7, the mean frequency offset is 1.9 # 10–14, with a k = 2 uncertainty of approximately 1.3 # 10–14 after one month of averaging. The lower and upper bounds of the coverage area over a one month interval would be 0.6 #10–14 and 3.2 #10–14, respectively, with respect to UTC(NIST). Note that the frequency uncertainty decreases as the averaging time increases. The estimated uncertainty after 1 day of averaging is near 5 #10–14. 9. Summ ary The NIST Time and Measurement and Analysis service makes the measurement techniques used for international comparisons between the world’s best timing laboratories available to any calibration lab or research facility. The TMAS offers a combined standard uncertainty (k = 2 coverage factor) of less than 15 nanoseconds for time, and less than 1 #10–13 for frequency after 1 day of averaging. The service is available though NIST as service number 76101S at a cost of $750 per month, with a onetime startup fee of $1500. [1] 10. A c kno wl e dg em e nts The authors thank Bob Graham of Sandia National Laboratories in Albuquerque, NM, for his assistance in beta testing a TMAS system, and for the use Vol. 1 No. 4 • December 2006 of the data shown in Fig. 7. We also thank the National Research Council in Ottawa, Canada for the use of the SIM data shown in Fig. 8. 11 . R e f e re n c e s [1] “NIST Calibration Services User Guide,” NIST Special Publication 250, (current copy is available on-line at http://ts.nist.gov /ts/htdocs/230/233/calibrations/). [2] S.R. Stein, G. Kamas, and D.W. Allan, “New time and frequency services at the National Bureau of Standards,” Proceedings of the 15th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting, pp. 17-27, December 1983. [3] M.A. Lombardi, “Remote frequency calibrations: The NIST frequency measurement and analysis service,” NIST Special Publication 250-29, 90 pages, June 2004. [4] M.A. Lombardi, A.N. Novick, J.M. Lopez, J.S. Boulanger, and R. Pelletier, “The Interamerican Metrology System (SIM) Common-View GPS Comparison Network,” Proceedings of the 2005 IEEE Frequency Control Symposium, pp. 691698, August 2005. [5] M.A. Lombardi, A.N. Novick, and R.M. Graham, “Remote Calibration of a GPS Timing Receiver to UTC(NIST) via the Internet,” Proceedings of the 2003 Measurement Science Conference, January 2003. [6] T. Jones, Splitting the Second: The Story of Atomic Time, Institute of Physics Publishing, Bristol, UK, pp. 123-126, 2000. [7] D.W. Allan, H.E. Machlan, and J. Marshall, “Time Transfer using Nearly Simultaneous Reception Times of a Common Transmission,” Proceedings of the 1972 Frequency Control Symposium, pp. 309316, June 1972. [8] D.W. Allan and M.A. Weiss, “Accurate Time and Frequency Transfer During Common-View of a GPS Satellite,” Proceedings of the 1980 Frequency Control Symposium, pp. 334-346, May 1980. [9] D.W. Allan, D.D. Davis, M.A. Weiss, A.J. Clements, B. Guinot, M. Granveaud, K. Dorenwendt, B. Fischer, P. Hetzel, S. Aoki, M.-K. Fujimoto, L. Charron, and N. Ashby, “Accuracy of International Time and Frequency Comparisons via Global Positioning System Satellites in Common-View,” IEEE Transactions on Instrumentation and Measurement, vol. IM-34, no. 2, pp. 118-125, June 1985. [10] W. Lewandowski, J. Azoubib, and W. Klepczynski, “GPS: Primary Tool for Time Transfer,” Proceedings of the IEEE, vol. 87, no. 1, pp. 163-172, January 1999. [11] C.D. Ehrlich and S.D. Rasberry, “Metrological Timelines in Traceability,” J. of Res. of NIST, vol. 103, no. 1, pp. 93-105, January-February 1998. [12] M.A. Lombardi and A.N. Novick, “Comparison of the one-way and commonview GPS measurement techniques using a known frequency offset,” Proceedings of the 34th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting, pp. 39-51, December 2002. [13] “IEEE Standard Definitions of Physical Quantities for Fundamental Frequency and Time Metrology – Random Instabilities,” IEEE Standard 1139-1999, prepared by IEEE Standards Coordinating Committee 27 on Time and Frequency, March 1999. [14] X. Yang, Y. Hu, Z. Li, X. Li, and X. Zheng, “An algorithm for a near realtime data processing of GPS commonview observations,” Chinese Astronomy and Astrophysics, vol. 27, no. 4, pp. 470480, October-December 2003. [15] The BIPM “Circular-T” reports are archived at: www.bipm.org [16] “ISO Guide to the Expression of Uncertainty in Measurement,” prepared by ISO Technical Advisory Group 4, Working Group 3, October 1993. [17] J. Levine, “Averaging satellite timing data for national and international time coordination,” Proceedings of the 36th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting, pp. 41-52, December 2004. [18] M.A. Weiss, “Long Term Effects of Antenna Cables on GPS Timing Receivers,” Proceedings of the 2000 IEEE Frequency Control Symposium, pp. 637-641, June 2000. [19] W. Lewandowski and C. Thomas, “GPS Time Transfer,” Proceedings of the IEEE, vol. 79, no. 4, pp. 991-1000, 1991. MEASURE | 59 REVIEW PAPERS Gravimetric Calibration of Volumetric Standards with Capacities Exceeding Five Gallons L .F. E a so n A b s t r a c t : Recently, the demand for volumetric measurements with greater accuracy and smaller measurement uncer- tainties has increased dramatically. In response, the metrology laboratories of the Arizona, Maine, Michigan, and North Carolina weights and measures programs have established gravimetric calibration capabilities for volume standards (provers) with capacities up to 100 gallons (500 liters). This collaborative effort with the National Institute of Standards and Technology (NIST), Weights and Measures Division (WMD), has improved volumetric prover calibration accuracy and uncertainty significantly. Accuracy is a measure of how close a measurement is to the actual value. Uncertainty is a measure of how well the value is known. Smaller uncertainties in laboratory standards have lead to similar improvements in field prover calibrations up to 2000 gallons. Prover calibration improvements facilitate better meter calibrations. Consequently, petrochemical terminals can have more confidence in inventory records, reducing inexplicable product losses. Apparent losses that once were ignored because they were less than the existing measurement uncertainty can now be investigated. Meters can be adjusted before loss totals increase. Gravimetric volume calibration makes use of existing mass comparator balance technology and mass standards commonly available in the State metrology laboratories. Any volume up to the limit of available mass standards and balance capacity can be calibrated without the expense of the multiple volume standard provers typically required for calibration of non-standard volumes. Thus, metrology laboratories benefit from significant cost savings in addition to improving their calibration process. Since gravimetric calibrations are traceable to mass, fewer laboratory volume standards must be calibrated by NIST, providing another cost savings to laboratories. Prior to large volume gravimetric calibration, volume transfer calibration uncertainties (k = 2) from NIST were reported at 130 ppm (3.1 cubic inches for a 100 gallon prover). Using this L. F. Eason uncertainty as a starting point, laboratories then had to add their own process North Carolina Department uncertainty factors and reported significantly higher uncertainties for field of Agriculture & Consumer Services provers. Using gravimetric calibration, a technician proficient in mass metrolStandards Laboratory ogy can expect to attain expanded uncertainties (k = 2) of 70 ppm (1.6 cubic 1051 Mail Service Center inches for a 100 gallon prover). These advances in gravimetric calibration Raleigh, NC 27699-1051 USA E-mail: LF.Eason@ncmail.net improve volume measurements at all levels, from the laboratory, to the terminal, and to the retail market. 60 | MEASURE www.ncsli.org REVIEW PAPERS 1. The Mark et Several market-based factors are driving the need for more accurate volume standard calibrations. With record increases in the price of petrochemical products, the potential cost of inaccurate measurements is greater than ever before. The cost of downtime needed for high volume petrochemical meter calibrations also continues to increase. Meter manufacturers have developed new metering systems with better temperature measurement and compensation systems to address the need for increased measurement accuracy. More efficient volume standards, such as dynamic small volume provers (SVPs) have been developed to decrease down time for meter testing (typically known as meter proving within the industry). These advances in measurement technologies have necessitated commensurate improvements in volume standard calibration technology. According to the March 2005 Petroleum Marketing Monthly, almost 712 million gallons1 of petroleum products were sold in the United States each day during the month of December 2004. [1] The average before tax U.S. refiner price for this product during the same time was $1.21(US) per gallon. Even at this price, petroleum sales exceed $861 million (US) per day. If December sales were typical of other months, that equates to over $314 billion (US) per year. This figure ignores tax revenues and retail profit margins. The price continues to rise. Obviously, measurement of this product is important so that measurement uncertainties should be minimized. 2 . M e te r To l e r a n c e s In the United States, commercial meter tolerances are set by the National Conference on Weights and Measures (NCWM). These consensus standards are published in NIST Handbook 44. Petroleum meters are covered as class 0.3 devices under the Liquid Measuring Device (LMD) code. [2] Lowering these tolerances for liquid measuring devices has been proposed as one way to mini1 The SI unit for volume is cubic meters, but for all practical realizations in the United States, gallons are used. Vol. 1 No. 4 • December 2006 6 5 7 1. Drain valve 2. Drain slope 5º 3. Levels 4. Level cover 5. Gauge mounting 6. Rolled bean 7. Top cone pitch 25º 8. Reinforcing bands 9. Bottom core pitch 20º 4 3 8 2 1 9 10. Adjustable legs 10 F i g u re 1. NIST Handbook 105-3 Field Standard Prover. [3] mize uncertainties. Several issues need to be considered before this course of action is taken. It is debatable if lowering the tolerances will have any effect on the market. In the author’s experience, few (if any) petroleum terminals allow meters to be in error by the NIST Handbook 44 tolerance of ±0.3 % for routine calibrations and ±0.2 % for new installations or repaired meters. It is common for terminals to require meter adjustments for errors that exceed either ±0.05 or ±0.03 %. Meter technology appears to allow these adjustments and meters repeat within this reduced allowance. However, what about the meter’s accuracy and the accuracy of the standards used to test these meters? Like any other measuring device, petroleum meters can only be as accurate as the standards that are used to calibrate them. Historically, these standards have been either mild-steel or stainless steel vessels. Test measures ranging from one to ten gallons are used to calibrate small capacity meters such as service station gas pumps. Provers ranging from 25 gallons to 2000 gallons are used to calibrate (prove) meters ranging from high capacity diesel pumps at truck stops, to large meters at fuel terminals. Very large capacity loop provers are used to calibrate the very high flow rate meters used at refineries and on petroleum pipelines. This paper will focus on the calibration of 25 to 2000 gallon volumetric provers used in the calibration of the midrange meters. Calibration of newer technology devices, known as dynamic small volume provers (SVPs) or captive displacement provers (CDPs), will also be discussed. For the remainder of this paper, these standards are referred to simply as provers and SVPs. 3 . Pr o v e r D e s i g n Provers typically are designed to meet National Institute of Standards and Technology (NIST) [3] or the American Petroleum Institute (API) design requirements. [4] The NIST and API design requirements are very similar (see Fig. 1). Provers are made of either carbon or series 300 stainless steel, both of which have well defined thermal expansion properties. Historically, provers have been calibrated by a volume transfer calibration MEASURE | 61 REVIEW PAPERS procedure. Water is transferred into the prover being tested from other NIST traceable, calibrated provers. API has also recognized water draw calibration, which is the reverse of the volume transfer. Instead of water being delivered from standards into the unit under test, the unit under test is filled to the nominal zero mark and drained into a calibrated prover. Water draw calibration has often been used as a field calibration technique for provers that are too large to transport to a laboratory for calibration. For volume transfer and water draw, the difference between the known volumes of the standards and the observed volume of the prover being calibrated is used to either calculate a calibration factor or adjust the prover under test as close to nominal as possible. Temperature corrections based on the temperature of the water and the coefficients of expansion for both provers are calculated. The calibrated volume is referenced at either 15 ºC or 60 ºF, depending on where the prover will be used. Both NIST and API procedures require provers to be large enough to hold the amount of liquid delivered from a meter at full flow for one minute. Since start up and stopping flow rates are reduced to prevent excessive vaporization, over flow, and foaming of the product, a prover actually has to be somewhat larger than the liters or gallons per minute delivered through the meter. For example, to deliver one minute at full flow, a meter delivering 400 gallons per minute of fuel oil, might need to deliver an additional 100 gallons at a slower flow to prevent excessive foaming at the start and end of the calibration. Therefore, using a 500-gallon prover would be marginal to test this meter even though it is larger than the volume delivered in a minute of full flow from the meter. The same prover may be acceptable for a meter of the same capacity delivering gasoline since the start up and slow down volumes are less for gasoline. Provers are typically calibrated “to deliver” a specified volume. This is necessary so that the prover doesn’t have to be completely dried between runs. Instead, a drain time is specified. The drain time is typically 30 seconds after cessation of main flow. This wet down 62 | MEASURE procedure is done at the time of calibration and every time the prover is used in the field. After the drain time, the drain valve is closed and the remaining product clinging to the walls of the vessel is allowed to remain in the prover. Thus, an assumption is made that there is a repeatable amount of fluid left in the prover at the beginning of each run. This necessitates a wet down run that is disregarded each time the prover is used. Establishing an accurate drain time is problematic for water draw procedures when multiple standards are required since the prover has to be stopped each time a standard is filled and the next standard is filled. 4. Sm al l Vo l u m e P ro v e r D e s i g n There are several problems with the use of traditional provers at terminal loading racks. These include: • In order to hold a full minute flow for high capacity meters, provers must be very large. • Provers are difficult to transport from one location to the next or to the laboratory for calibration. • Since provers are large, they require the lane of meters being calibrated to be shut down for extended lengths of time. • Meter testing at multiple flow rates is very limited due to the time requirements to fill the large volume. • Even during a “full flow test,” some percentage of the volume is actually pumped at a slower rate for start-up and slow-down. Thus, the meter factors calibrated for full flow are always slightly skewed away from the actual full flow factor. This can be significant for kerosene and fuel oil distillates, since the volume metered at slow flow may be up to 25 % of the total volume. • Since the percentage of slow flow to full flow will vary between provers of different volumes, different meter factors will be calculated, depending on the nominal volume of the provers being used. • There is a possibility of spills from overflow when a meter is significantly out of calibration. • Since a traditional prover is not a completely closed system, vapor loss may be a consideration. • Drain times often differ considerably depending on the product and the pump back system. Differences in drain times affect the wet down condition of a prover and the calibration by an unknown amount. To minimize these problems, the dynamic small volume prover was developed. [5] Rather than a tank of known volume, the SVP is a displacement type prover. A piston travels with the flow of liquid through a cylinder (see Fig. 2). The calibrated volume of the cylinder is defined by two optical switches that read a flag traveling with the piston. The meter test begins as the flag on the piston rod passes the first optical switch and stops as it passes the second calibrated switch. At the end of the calibrated length, a poppet valve opens to let product flow by with minimal obstruction. The piston is then pulled back to a staging position before the first optical switch to start the process over again. Between these two switches, the computer on the SVP counts the pulses generated by the meter. This counter uses two timers that are slightly offset. At the end of the calibration run, the pulse count is interpolated between the two timers using a technology called double chronometry pulse interpolation. [6] This allows a much more precise count of pulses than the traditional pipe or ball prover technology. This procedure is repeated a number of times (typically five) at each flow rate. The interpolated pulse values are compared to verify the repeatability of the meter. If all repeat within a specified percentage (typically ±0.02 % of volume), the run is approved, the interpolated pulse counts are averaged, and a meter factor is calibrated for that specific flow rate. The meter flow rate capacity versus the SVPs calibrated volume is very large compared to the traditional graduated neck prover. Depending on the manufacturer and model, SVPs may be rated for flow rates up to 100 times its calibrated volume. For example, a SVP with a fifteen-gallon calibrated volume is rated at 1497 gallons per minute. It would take a 2000 gallon graduated neck prover to calibrate a meter with the same flow rate. Though the SVP tends to minimize www.ncsli.org REVIEW PAPERS Seal Monitor Sensor On-Board Microprocessor Optical Detector Switches Measuring Section Invar Rod Piston Guide Displacer Actuator Launch Spring Main Displacer Power Unit Differential Pressure Switch for Monitoring Bypass Seal Integrity Main Displacer Seals Guide Tracks Guide Rods Outlet Flange Bypass Valve Balance Valve Vertical Lift Stop Inlet Flange Bypass Actuator F i g u re 2. Small volume prover from NIST Handbook 105-7. [3] most of the problems identified with traditional graduated neck provers, there are issues unique to this technology. With this high flow rate to calibrated volume ratio, it is obvious that the calibration of the SVP is very critical. A small uncertainty in a graduated neck prover calibration has much less affect than the same uncertainty on a SVP calibration. This is one of the primary reasons that the metrology laboratories of several State Weights and Measures programs are moving to establish improved gravimetric measuring capabilities for SVPs and larger volume standards. Other factors that affect comparisons between meter calibrations in the field using SVPs and graduated neck provers are beyond the scope of this paper. 5. Tr a d i t i o n a l C a l i b r a t i o n P ro c e d u r e s As with the calibration of meters, the calibration of provers is limited by the quality of the calibration standards used. Traceability to the national standard is the ultimate goal. Since volume is Vol. 1 No. 4 • December 2006 derived from mass, in the United States this chain of traceability must eventually extend back to the national prototype kilogram 20 (K 20) at NIST. This platinum iridium mass standard is used to calibrate multiple levels of mass working standards. These are then used to propagate the unit of mass to both larger and smaller working standards by using complex weighing designs. At NIST, some of these working standards are then used to gravimetrically calibrate small volume standards up to the fivegallon level. (Gravimetric calibration is covered in detail later in this paper.) Historically, provers larger than 20 liter or five gallon were calibrated by volume transfer, using multiple drops from a well characterized, gravimetrically calibrated five-gallon slicker plate standard. The weighing equipment available at the time, a large equal arm balance, imposed this limitation. A 100-gallon NIST working standard was also calibrated by volume transfer and used to calibrate 100-gallon and larger customer standards. With this procedure, the reported NIST volume calibration uncertainty (k = 3) was ±0.02 % of the volume. This resulted in an uncertainty of ±100 milliliters per 500 liters or ±4.62 cubic inches per 100 gallons. Since each link in a calibration chain adds to the measurement uncertainty, this ±0.02 % was just the starting point for additional calibrations. In the United States, NIST performs relatively few large volume calibrations compared to the next level in the national measurement chain. The majority of large volume calibrations are delegated to private calibration laboratories and the State metrology laboratories. According to the 2003 NCWM State Laboratory Program Survey, in 2002, the State metrology laboratories calibrated 6966 test measures, 1053 provers, and 555 glassware standards. The state metrology laboratories are coordinated and evaluated by the NIST Weights and Measures Division (WMD). In addition to NIST WMD recognition, many of the state metrology laboratories are now NIST National Voluntary Laboratory Accreditation Program MEASURE | 63 REVIEW PAPERS (NVLAP) accredited. Both of these programs are based on ISO/IEC 17025 requirements. Each additional step in the traceability chain between NIST and a prover being calibrated adds an additional level of uncertainty to the measurement. Thus, volumetric transfer calibrations from these labs started with a ±0.02 % uncertainty for the standards calibrated at NIST. Each laboratory added a level of uncertainty based on their process uncertainty to the calibration of a customer’s prover. NIST WMD has circulated a 100gallon stainless steel prover for interlaboratory comparisons among State metrology laboratories since 1988. The average calibration uncertainty reported by states between 1988 and 1993 was ±7.0 cubic inches or approximately ±0.03 % of the volume. NIST Handbook 44 [2] and virtually all quality programs require the total standard and calibration process uncertainty to be less than one third of the tolerance of the device under test. Starting with a standard uncertainty of ±0.03 %, even if the process uncertainty of the meter calibration were negligible, the provers would not be adequate for meter test tolerances below ±0.1 %. This is double the tolerance that the terminal loss control engineers use. Many additional factors affect meter calibration in a field calibration. These include: • Temperature measurement • Scale plate readability • Steel coefficient of expansion uncertainty • Evaporation • Vaporization • Pressure surges from other trucks filling • Differences in pump off time from calibration drain time • Prover drain characteristics • Operator training • Meter reading sensitivity • Prover leveling • Various other human errors When these factors are all combined, it is easy to identify another ±0.1 % uncertainty in the meter calibration process itself. At this level of uncertainty, it was difficult to achieve the NIST Handbook 44 tolerances, much less the ±0.05 % uncertainty required by the terminals. Improvements were needed. 64 | MEASURE 6. I m pro v e m e n t s a t N I S T Recognizing the need for improvement, API worked with NIST in the early 1990’s to improve the measurement capabilities of the NIST volume laboratory. In addition to facility improvements, a new 600-kilogram capacity mass comparator was purchased to allow gravimetric calibration up to 500 liters or 100 gallons. In a gravimetric calibration, the water in a vessel is weighed. The volume is then calculated based on the density of the water with suitable corrections for temperature and air buoyancy. Thus, an entire step in the traceability measurement chain is eliminated. A customer’s 100-gallon prover could be calibrated directly from mass working standards. In addition, the NIST 100gallon working standard prover was calibrated gravimetrically and housed in a climate controlled laboratory. When this system was brought on line in 1994, the NIST volume calibration uncertainty dropped from ±0.02 % to ±0.004 %. Volume transfer calibration uncertainties from the 100-gallon working standard also improved both from the lower starting uncertainty of the 100-gallon working standard and the improved environment used for the calibrations. This was a very large step forward for volume measurement in the United States. Another benefit realized from this improvement was that the technology spread to the State metrology laboratories, including the North Carolina (NC) Standards Laboratory. The large volume gravimetric calibration procedure uses equipment, standards, and procedures that were already in many of the state metrology laboratories. According to the 2003 NCWM State Laboratory Program Survey, in 2002 nearly 80 % of the 375,000 standards tested by the state metrology laboratories each year are mass standards. With this workload, the state laboratories have focused on maintaining their mass calibration capabilities. To meet NIST WMD and NVLAP mass calibration requirements, these laboratories have well controlled laboratory environments, well-characterized stateof-the-art mass comparators, very precise environmental parameter measurement instrumentation, and well trained metrologists. As gravimetric calibration techniques were advanced at NIST, the state laboratories were in a good position to adopt the same procedures. The NC Standards Laboratory had been using gravimetric calibration for all volumes up to 20 liters and five gallons with excellent results. It was a relatively easy step to increase the range to 100 gallons. 7. Gr a vi m et ri c Ca l ib ra ti on The original National Bureau of Standards (NBS, changed to NIST in 1988) gravimetric procedures for large volume standards were written for equal arm balances. These procedures were very cumbersome, requiring transposition or double substitution weighing of the volume standard empty, filled, and drained. Each measurement required different mass standards and was very time consuming. With the filled vessel supported on the hanging pan of the balance, there were many opportunities for spills and damage to standards. A source of distilled or deionized water had to be near the balance. If water dripped on the outside of the standard, it had to be carefully dried off to not affect the measurement. Due to the time it took, evaporation was a significant problem. Gravimetric calibrations above one liter were uncommon. The advent of the single pan electronic balance improved gravimetric calibration possibilities. As precision electronic mass comparators increased in capacity and resolution, gravimetric calibrations up to 20 liters or five gallons became much faster and easier. NBS Handbook 145, SOP 14, Recommended Standard Operations Procedure for Gravimetric Calibration of Volume Ware Using an Electronic Balance [7], was a practical procedure for gravimetric volume calibration. Any laboratory with an adequate electronic balance, appropriate mass standards, a source of distilled or deionized water, and a metrologist trained in precision mass measurement, could calibrate volume standards gravimetrically. A complete set of calibrated metric (15 milliliters to five liters) and avoirdupois (120 minims to one gallon) glass standards were issued to each state laboratory by NBS in the 1960’s. These were automatic burettes and pipettes primarily www.ncsli.org REVIEW PAPERS -0.30 -0.35 Error (Cubic Inches) -0.40 -0.45 -0.50 -0.55 -0.60 -0.65 -0.70 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year F i g u re 3. Gravimetric Calibration of North Carolina 5-Gallon Working Standard. used to test glass standards for regulatory Weights and Measures field inspectors. Regional interlaboratory comparisons using both volume transfer and gravimetric calibration comparisons were completed on glassware and fivegallon test measures. Laboratory Auditing Problem (LAP) 28 was initiated to monitor volume calibration capabilities within the state laboratories. It was at this time that the North Carolina Standards Laboratory started using gravimetric calibration for all volume calibrations for standards up to four liters, or one gallon. There were several reasons for this change. There were significant safety concerns with cleaning the glassware standards with concentrated sulfuric acid, which was drawn into the standard with vacuum. In addition, the glass volume standards used by the lab were beginning to show signs of age with chipped tips and drain times different from the original NBS calibrations. Since these aging volume standards had been calibrated at NBS gravimetrically, gravimetric calibration eliminated a step in the traceability chain for customer glassware standards. Changing to gravimetric calibration avoided these problems. Also, expense and risk of shipping damage were decreased by not having to send volume standards back to NIST for calibration. A 1990 regional interlaboratory comparison revealed a 0.4 cubic inch bias in five-gallon calibrations made by the North Carolina Standards Laboratory. An internal gravimetric calibration elimVol. 1 No. 4 • December 2006 inated this bias. From that time on, the one gallon, five-gallon and 20-liter slicker plate reference standards in the NC laboratory have been calibrated gravimetrically. The accuracy and repeatability of these calibrations have been verified through years of history (see Fig. 3) and multiple interlaboratory comparisons. 8. G r a v i m e t r i c Wa t e r D r a w Concept In 1997, the NC 100-gallon stainless steel prover was recalibrated by NIST. One of the NC metrologists delivered the prover and assisted John Houser with the gravimetric calibration. From that point, NC began planning for large volume gravimetric calibration. However, it was requests for SVP calibrations that were actually the driving force behind the development of our large volume gravimetric calibration procedure. With increasing number of requests for SVP calibrations, the NIST Weights and Measures Division decided to work with North Carolina, Arizona, and Michigan to establish gravimetric SVP calibration capabilities on a regional level. The collaboration between the NIST Weights and Measures Division and the Arizona, Michigan, and North Carolina State metrology laboratories, has answered many questions and facilitated significant improvements in gravimetric calibration procedures. The Maine State metrology laboratory metrologist developed a program in parallel with this initiative. Indiana Weights and Measures Laboratory metrologists also calibrate SVPs but use a procedure based on volume transfer water draw into multiple five-gallon test measures. Much has been learned about the limiting factors in gravimetric calibration as these programs have developed. The water draw procedure, as described by API in the Manual of Petroleum Measurement Standards [8], is a volume transfer procedure. However, the procedure lends itself well to gravimetric calibration. The objectives of revising the API procedure included: • To minimize measurement uncertainties by limiting the procedure to the actual measurement of the mass of the water delivered by the vessel, rather than subtracting the measured mass of the "wet" vessel from that of the filled vessel. • To minimize the effect of balance nonlinearity by testing the balance sensitivity at the nominal mass of water being weighed at the time of calibration. • To minimize the effect of balance drift by allowing the balance to be zeroed immediately before each measurement. • To minimize the effect of temperature changes by shortening the duration of the procedure. 9. G r a v i m e t r i c Wa t e r D r a w P ro c e d u r e The nine general steps of the weighing procedure are summarized below. 9 .1 De t e r m in e St a n d ar ds t o Us e Based on the nominal volume of the prover to be calibrated (hereafter referred to as unit under test or UUT), calculate the approximate mass of that volume of water. Choose mass standards to use so that the mass of the standards approximately equals the calculated water mass. The difference between the calculated water mass and the mass of the standards should not exceed 0.05 % of the calculated water mass. The mass of this combination of standards will be referred to as Ms. For non-metric volumes, it may be more efficient to add tare weights (tx) to the calculated mass of the water so that the mass of water plus tx equals a conMEASURE | 65 REVIEW PAPERS venient combination of mass standards for Ms. Effort should be made to both minimize the number of weights that must be handled and make sure that Ms is as close to the calculated water mass as practical. 9 .5 R e c o r d A m b i e n t C o n d i t i o n s f o r A i r D e ns it y C a l c ul a t io n 10. G r a v i m e t r i c Wa t e r D r a w Ad vant ages Record the air temperature, barometric pressure, and relative humidity readings to use for air density calculation. In a traditional gravimetric calibration, the unit under test is weighed drained and then filled. To establish traceability, each weighing is a comparison to standards with mass values nominally equal to the weight of the wet down standard and then the standard filled with water. The new single pan electronic mass comparators make this much easier than the old equal arm balances, but it still can be very awkward to move and position the prover being tested on the balance pan. In the case of SVPs, it is impossible. Filling the standard while it is on the balance also introduces chances for error and damage to the balance from spilled water. In addition, the combinations of weights needed to nominally equal the mass of the prover being tested and then the filled prover can only be determined after weighing the standard. Often, the necessary combinations of weights are cumbersome to use. In a volume transfer water draw, water is drained from the UUT into a standard or series of standards equal to the nominal volume. If this requires multiple standards, the drain time of the prover being tested must be interrupted each time a standard is filled and replaced. In addition to distorting the drain time, the multiple transfers add opportunities for mistakes. Since SVPs are displacement provers, they have to be water drawn. They also have volumes different from typical standard provers. In order to calibrate a variety of SVPs with a single drop, a laboratory would have to have 15, 20, 30, 40, 65, 75, 120, and 170gallon provers. It would be very difficult for most labs to purchase and maintain such an inventory of provers. A gravimetric water draw can take advantage of the best features of each process. The standard provers can be replaced with a simple transfer vessel that is large enough to contain the full volume of the prover being calibrated. This vessel only serves to contain and move water from the unit under test to the balance and to the drain. Therefore, drain characteristics or the physical properties of the material are not important. Neither does it have to be the exact volume of the prover being tested. One 9 .6 Pr e p a r e t h e U U T 9.2 We i g h t h e Tr a n s f e r Ve s s e l to Det er mi n e Appro pr i a t e Ta re We i g h t s To establish the wet down condition of the transfer vessel, fill it with tempered distilled or deionized water above the lowest point of the drain hole. Open the drain and let this water drain out. Close the drain and place the transfer vessel on balance. This reading indicates the approximate mass that will need to be added to the balance and tared off before the mass standards are placed on the balance. This mass will be referred to as tare zero (t0). The t0 tare weight is used to duplicate the approximate weight of the transfer vessel. Like the transfer vessel, the t0 weights will be zeroed off and duplicate the effect of zeroing off the transfer vessel. They will not be used in any calculations. This step insures that the mass of the water and mass of the standards are both measured within the same sensitivity range on the balance. This avoids sensitivity uncertainties that could be caused by balance nonlinearity if the standard and the water were weighed in different parts of the balance electronic weighing range. 9 .3 F i l l t h e U U T i n P r e p a r a t i o n f or C al i br at i o n Fill the UUT to the nominal volume indication with distilled or deionized water that is near ambient room temperature. 9.4 We i g h t h e M a s s S t a n d a rd s fo r Readi ng O1 Place the tare zero (t0) weights on the balance. Zero the balance. Place predetermined mass standards closely approximating the nominal mass of the water volume to be measured on the balance. Record this balance reading as O 1. Remove the mass standards from the balance. 66 | MEASURE Take the temperature of water in the UUT. Verify that the UUT is still filled to the nominal indication and adjust if necessary. This step should be repeated immediately before draining into the transfer vessel. 9.7 Ta re Off Tr a n s f e r Ve s s e l Place the transfer vessel and lid on the balance. Zero the balance. 9 .8 W e i g h W a t e r f o r R e a d i n g O 2 Immediately after zeroing the balance, remove the transfer vessel and take it to the unit under test. Drain the water from the UUT into the transfer vessel. Drain for 30 seconds after cessation of main flow unless another drain time is specified. Place the lid on the transfer vessel to limit evaporation. Carefully return the transfer vessel to the balance. If needed, add known tare weights (tx) to the balance pan. This balance reading is recorded as O2. 9 .9 R e p e a t P ro c e d u re Repeat procedure until confidence has been established in the measurement. All repeated runs must agree within ±0.02 %. Determine the mean of the results. It should be noted that this revised procedure primarily applies to vessels calibrated “to deliver”. When a vessel is calibrated “to deliver”, the interior is wetted down with the liquid to be measured by a standard procedure immediately before each use. Typically, for small, hand held measures, this “wet down” requires a uniform 30-second pour and a 10-second drain after it is empty. For larger vessels, the filled vessel is allowed to drain at full flow and then allowed to drain an additional 30 seconds after cessation of main flow. The advantage of calibrating a vessel to deliver is that it can be used without drying each time between uses. In order to calibrate the vessel “to contain” using this procedure, the dried vessel must be used as the transfer vessel. www.ncsli.org REVIEW PAPERS 11. Wa t e r P u r i t y F i g u re 4. Transfer vessels, pallet stacker, and mass comparator. transfer vessel will work for a range of calibrations. It is just important for the transfer vessel to be sturdy, not leak, have a lid to limit evaporation, and to be stable when placed on the balance pan. There also has to be an efficient way to transport it to and from the balance. The North Carolina Standards Laboratory found two inexpensive, plastic, horizontal agricultural tanks that work well for this job (see Fig. 4). One is 35 gallons for provers between 15 and 30 gallons. The other is 125 gallons for larger provers up to 100 gallons. These tanks were banded on wooden pallets so that they can be moved with a pallet stacker. Other laboratories have a series of hooks and cables to lift and move the tanks with an overhead crane. This seems to minimize water movement within the tank and improve balance stability. Regardless of the method of transport, it is important that the movement be smooth to mini- mize sloshing of the product. In addition, there must be adequate control to slowly lower the vessel and center it on the balance pan to avoid damage to the mass comparator. Another major factor in the accuracy and repeatability of any gravimetric procedure is timing. The procedure has to be completed before there are any significant changes in the water conditions from when the unit under test is filled and adjusted to nominal. It is also important that conditions affecting the balance do not change between weighing the mass standards and weighing the water. The procedure should be carried out in a well-controlled environment. NIST Handbook 143 lists the requirements in Table 1 for gravimetric volume calibration. [9] More will be said about the ambient environment in the water temperature measurement section. Procedure Temperature I Gravimetric 20 °C to 23 °C, set point ± 2 °C Maximum change 1.0 °C/hr. Relative Humidity (Maximum Range per 4 hours) 40 % to 60 % (50 % ± 10 %) Ta bl e 1. Requirements for gravimetric volume calibration from NIST Handbook 143. Vol. 1 No. 4 • December 2006 The purity of water in the traditional volume transfer volume calibration is of little consequence. Water needs to be clean, but not necessarily pure. Of course, these relative terms need definition. As a rule, potable water is clean enough for volume transfer calibrations. The purity of water is not as critical in volume transfer since any uncertainty in density measurement it causes, will be applied equally to both the unit under test and the standard itself. As long as nothing is done to change the density of the water between vessels (such as a major temperature change), density uncertainties cancel each other out. Excessive air trapped in the water is a problem in volume transfer calibration however since it bubbles out over time. Traditional gravimetric procedures such as NIST Handbook 145, SOP 14, specify ASTM D 1193 Type IV water. In gravimetric calibration, the water density is critical in two ways. First, an accurate water density is necessary to calculate the mass of the water. There is a significant difference between the density of the mass standards (typically between 7.3 grams per cubic centimeter and 8.0 grams per cubic centimeter) and water (approximately 1.0 gram per cubic centimeter). Therefore, the air buoyancy correction is very significant to the mass calculation. Second, the water density is the only factor used to convert from measured mass to volume. Regardless of the accuracy and precision of the mass determination, the volume conversion is proportionally dependant on the accuracy of the water density. A 1 % uncertainty in water density will result in an approximate 1 % uncertainty in volume. A 0.001 % uncertainty in water density will result in a one milliliter uncertainty per 100 liters. There are two ways to determine the water density. The first is to measure it directly. Commercial density meters are available that read out to five or six decimal places. Second, if an adequate density meter is not available, water density can be estimated based on an assumption of water purity and precise measurement of the temperature. Work by Gary Cohrs at Calibron Systems, Inc. [10] using Archimedes’ principle and a MEASURE | 67 REVIEW PAPERS 500 Gal lo n P ro v er Wa t e r Te m p e r a t u re D a t a F irs t 60 M in u te s Temperature (degrees C) 17.50 17.00 16.50 Mid Scale Nec 16.00 Top of Cylinde Middle of Cylin Bottom of Cyli Ambient 15.50 0 10 20 30 40 50 60 Minutes F i g u re 5. DI water filter, resistivity meter, and pump. 500 Gal lo n P ro v er Wa t e r Te m p e r a t u re F irs t 5 Da ys 17.50 68 | MEASURE 21.50 Temperature (degrees C) Temperature (degrees C) vibrating tube density meter, led him to conclude that either deionized (DI) or 17.00 distilled water should be used for gravimetric prover calibration. Based on the assumption that the 16.50 only pollutants in tap water meeting federal drinking standards that have a measurable affect on density are dissolved16.00 salts, Mr. Cohrs developed a relationship between city tap water density and conductivity. However, this estimation has not yet been repro15.50 0 10 ducible in other laboratories and adds an uncertainty factor that can be avoided by using either DI or distilled water. The North Carolina Standards Laboratory purchased the DI water filtration system shown in Fig. 5. Using a resistivity meter immediately after DI resin beds, water with a resistivity of 18,000,000 ohm (18-megohm) was easily achievable. However, the resistivity rapidly degraded when put in a storage tank (a resistivity of 2-megohm immediately after storage) or when piped through schedule 80 PVC piping (6 to 7megohm). This 18-megohm water had not been exposed to any significant sources of contamination. Was the drop in resistivity significant to the density of the water? To answer this question, Georgia Harris of the NIST Weights and Measures Division collected water samples from various labs around the country. She tested all of these using a five-digit 23.50 19.50 Mid Scale Neck (800 cu in @ Zero) Top of Cylinder Middle of Cylinder Mid Scale Neck (100 cu in Bottom of Cylinder 17.50 Top of Cylinder Ambient Middle of Cylinder 20 30 15.50 40 50 Bottom of Cylinder 60 Ambient Minutes 0 1000 2000 3000 4000 5000 6000 7000 Minutes F i g u re 6. Water temperature equilibration. water density meter. Samples of fresh DI water, distilled water, reverse osmosis filtered water (RO), deionized reverse osmosis water (RODI), and tap water samples were collected. Some of the samples had been stored for significant lengths of time and some were recycled water that had been used in previous gravimetric calibrations. Though analysis of her data is still ongoing, initial analysis indicates that DI, distilled, and RODI water from the state metrology laboratories in Arizona, Maine, Michigan, and North Carolina, the NIST volume group, and the NIST Weights and Measures Division training labs are all equivalent. There were no significant differences in density, regardless of the age of the sample. On the other hand, the densities of RO water and tap water from the same locations differed significantly. The preliminary conclusion is that if DI or distilled water is initially pure (verified by a conductivity or resistivity meter) and is stored properly, density does not change with normal storage. The appropriate water density formulas can still be used to estimate the density based on temperature. Ms. Harris also determined that reverse osmosis does not always filter out enough of the contaminants for the water to have a predictable density. If www.ncsli.org REVIEW PAPERS F i g u re 7. 1000-gallon water storage tank. RO water is used, it should also be deionized. 12. Wa t e r Te m p e r a t u r e M e a s u re m e n t Accurate measurement of the water temperature is critical to gravimetric calibration. An uncertainty of 1 ºC at 24 ºC will result in a calibration uncertainty of 0.02 %. It is relatively easy to purchase a thermometer with an accuracy of 0.1 ºC or better, but this does not solve the problem completely. There are two other major factors to consider. These are the stability and uniformity of the temperature in the standard (gradients). Additional precautions need to be taken to limit these potential sources of uncertainty. The stability of the water can be increased by storing for an extended period in the temperature-controlled climate where the calibration will take place. Storage capacity should include enough water to complete the calibration. Temperature equilibration requires much time. Any difference between the ambient temperature and the water temperature results in temperature changes during the calibration as the water seeks equilibrium with room temperature. The uniformity of the water temperature is also critical to the measurement. To analyze this, a 500-gallon stainless steel prover was filled with tap water. The results are shown in Fig. 6. In this test, water temperatures were measured Vol. 1 No. 4 • December 2006 tank. If the flow of water is stopped, the at four different levels in the prover. pressure increases to a point that water These were the center of the neck scale bypasses the system back up to the (the line with squares), top of the cylinstorage tank rather than circulating in der, middle of the cylinder, and bottom of the pump. Another option would be to the cylinder (lines with triangles and diamake sure the pump used is a magnetic monds). The dashed line is the ambient drive pump so that when flow stops, the room temperature, which is off scale on motor is allowed to turn relatively freely the graph covering just the first hour. All without heating up. Finally, if the storage measurements were taken in the centertank is not at the same level or in the line of the provers, straight down same location as the prover calibration, through the neck opening. Rapid care must be taken so that the temperaresponse bare wire bead thermistors were ture in the area of the tank is the same as used as temperature sensors. the calibration area. In North Carolina Several things are clear from the data. where the storage tank is on a warmer It takes a considerable amount of time mezzanine, fans were added to circulate for the water temperature to equilibrate. and mix the air properly. Figure 7 shows The time would have been even longer if the 1000-gallon storage tank with one of there had been more of a difference the circulating fans in the background. between the water temperature and ambient. In addition, as expected, the neck temperature changes much faster 1 3. E q uat io n s than the main body of water. This data illustrates the importance of measuring 1 3 .1 W a t e r D e n s i t y temperature in the center of the vessel There have been several water density and working quickly. In this example, the tables and equations over the past few fluid in the neck represented less than years. Currently the formula used by one percent of the prover volume. Since NIST, and with the most international the fluid level is gauged in the neck, it is acceptance, is the Patterson/Morris 1994 important that it not be significantly difwater density, ρ(tw), equation as shown ferent from the rest of the water. Again, below. [11] this points out the advantage of allowing 2 ρ (tw ) = ρ 1− A(t − t (1) ) + B( t − t ) + C ( t 0 0 the water to reach temperature equilib0 rium with the room before calibrating 2 3 4 provers gravimetrically. As discussed ρ (tw ) = ρ 1− A(t − t ) + B( t − t ) + C ( t − t ) + D( t − t ) + E( 0 0 0 0 0 previously, in North Carolina, a 1000gallon vertical plastic agricultural tank 2 3 4 5 (tw ) = ρ to 1−store − t ) + B( − t ) + C ( t − t ) + D( t − t ) + E( t − t ) was ρ purchased DIt water A(t water. 0 0 0 0 0 0 store well. Resistivity did not seem to increased rapidly during storage and piping. This made it difficult to prove the water was still of adequate quality. Therefore, water is stored as city water before it goes through the DI beds. This temperature equilibrated water is pumped directly from storage, through the DI filters, into the vessel being calibrated. This way the water is at the peak condition for both temperature and deionization. Two more temperature obstacles must be overcome. Care must be taken so that the pump used does not heat the water. This issue can be resolved by increasing the supply pipe diameter to the pump and piping a bypass line back to the tank. The bypass should be controlled by a pressure valve where it returns to the where: ρ0 = 999.97358 kg/m3 A = 7.0134 x 10–8 (ºC)–1 B = 7.926504 x 10–6 (ºC)–2 C = – 7.575677 x 10–8 (ºC)–3 D = 7.314894 x 10–10 (ºC)–4 E = – 3.596458 x 10–12 (ºC)–5 t0 = 3.9818 ºC tw = temperature of the water in ºC. Though the differences between equations are small and usually insignificant to the measurement uncertainty, there are differences. In our global economy, international acceptance and agreement are essential. MEASURE | 69 REVIEW PAPERS Typical Uncertainty Components for Gravimetric Water Draw Variable Description Components Uncertainty Estimate Type of Distribution Ma Molar mass of the air within laboratory: 28.963 5 x 10-3 kg/mol Std. Dev. Process, s(p) 34 ppm Normal Pooled Standard Deviation p Ambient barometric pressure in Pascal Water Temperature 6.3 ppm Rectangular Thermometer Uncertainty is 0.0052 ºC – increase factor times 10 for gradients T Ambient temperature in Kelvin Water Density 5.5 ppm Rectangular NIST, Jones/Harris 1992,10 ppm Air Temperature 0.43 ppm Rectangular Doubled water temp Barometric Pressure 0.40 ppm Rectangular Specifications from ability to measure pressure Relative Humidity 0.31 ppm Rectangular Specifications from ability to measure relative humidity Mass Calibrations 0.29 ppm Normal R h Universal gas constant: 8.314 510 J mol-1 K-1 Relative humidity in % 1.000 62 + (3.14 x 10-8) p + (5.6 x 10-7) t2 f t ambient temperature in degrees Celsius ρsv 1 Pascal x exp (AT2 + BT + C + D/T) A 1.237 884 7 x 10-5 K-2 B –1.912 131 6 x 10-2 K-1 C 33.937 110 47 103 –6.343 164 5 x a0 1.581 23 x 10-6 K Pa-1 a1 a2 –2.933 1 x 34.9 ppm Combined Uncertainty (k = 1) 69.8 ppm Expanded Uncertainty (k = 2) K Pa-1 1.104 3 x 10-10 K-1 Pa-1 b0 5.707 x 10-6 K Pa-1 b1 –2.051 x 10-8 Pa-1 c0 1.989 8 x 10-4 K Pa-1 c1 –2.376 x 10-6 Pa-1 1 3 . 2 A i r D e ns i ty The air density, ρ, should be calculated using the International Committee for Weights and Measures (CIPM) air density equation formula. [12] pMa ρ = ZRT (1 − 0.3780 xv ) | MEASURE Compute the volume, Vt , for each determination using the equation: M ρ 1 s a 1 – ρa – M(3) Vt = O2 t 1– ρs ρt ρw – ρa O1 ( (d + ) exv2 Variables for CIPM air density equation are shown in Table 2. Ta bl e 2. Variables for CIPM air density equation. 13.3 Vo l u m e a t C a l i b r a t i o n Wa t e r Te m p e r a t u re ( T w ) ρ ρ 1 Ms xv = ( h / 100 ) f psv= O (2) 1 – ρa – M t 1– a Vt 2 ρs O1 ρt ρw – ρa p p2 a + a t + a t 2 + ( b + b t ) x + (where: Z =1− c0 + c1t ) xv2 + 2 d + exv2 0 1 2 0 1 v T O1 = ObservationT#1, balance reading p p2 -11 2 2 Pa-2 a 1.83 Z = 1d − + a1tx 10 + a2t K + ( b0 + b1t ) xv + ( c0 + c1t ) xv2 + 2 0 T T e –0.765 x 10-8 K2 Pa-2 70 Mass uncertainty from standards calibration report (k = 1) Ta bl e 3. Uncertainty components of gravimetric water draw to be included in the uncertainty statement. D 10-8 Comments ) for mass standard O2 = Observation #2, balance reading for water delivered from vessel Ms = Mass of mass standard(s) Mt = Mass of tare weight(s) ρa = Air density ρs = Density of mass standard(s) ρt = Density of tare weight(s) ρw = Water density www.ncsli.org REVIEW PAPERS Additional Uncertainty Components for Small Volume Provers Uncertainty Type of Estimate Distribution Water Compressibility 1.8 ppm Rectangular NIST, Jones/Harris 1992, 10 ppm Prover Temperature 2.5 ppm Rectangular Temperature from thermometer well. Transference between water and metal is not good – double the thermometer uncertainty Detector Temperature 0.17 ppm Rectangular Temperature from bar. Transference between water and metal is not good - double the thermometer uncertainty Prover Pressure 0.025 ppm Rectangular Uncertainty @ 60 psi from calibration certificate Area Thermal Expansion (Ga) 0.58 ppm Rectangular Estimate @ 1 % of number given by calibration report, cubical coefficient is 2 % Detector Thermal Expansion (Gl) 0.051 ppm Rectangular Estimate @ 1 % of number given by calibration report, cubical coefficient is 2 % Mod. of Elasticity (E) 0.089 ppm Rectangular Estimate @ 1 % of number given by calibration report, cubical coefficient is 2 % 0.00074 ppm Rectangular Estimate accuracy to last number in value by one division 0.0010 ppm Rectangular Estimate accuracy to last number in value by one division Components Flow Tube Diameter (ID) Tube Wall Thickness (WT) Comments 35.0 ppm Combined Uncertainty (k = 1) 70.0 ppm Expanded Uncertainty (k = 2) Ta bl e 4. Additional uncertainty components for small volume provers to be included in the uncertainty statement. 13.4 Vo l u m e a t a R e f e r e n c e Te m p e r a t u re 1 3. 4. 2 Co m put ati o n o f V 60 Compute the volume at 60 °F, V60 , for each determination using the expression: 1 3. 4. 1 Co m put at io n o f V20 Compute the volume at 20 °C, V20 , for each determination using the expression: V20 = Vt [1 – α (t w – 20)] (4) where α is the cubical coefficient of expansion (1/°C) of the container being calibrated, and tw is the temperature of the water for each determination (in °C). Repeat for each determination and calculate the mean, l V20, for the duplicate measurements. Vol. 1 No. 4 • December 2006 V60 = Vt [1 – α (t w – 60)] (5) where α is the cubical coefficient of expansion (1/°F) of the container being calibrated, and tw is the temperature of the water for each determination (in °F). Repeat for each determination and calculate the mean, l V60, for the duplicate measurements. 14. Unc er tai nty Fa ctor s Traceability is not complete without an uncertainty statement. Working with Georgia Harris (NIST), Kelleen Larson (Arizona), Craig VanBuren (Michigan), and Bill Erickson (Michigan), the components of gravimetric water draw uncertainty factors listed in Table 3 have been identified. Additional factors that need to be considered for a small volume prover calibration are listed in Table 4. Most of these are based on information specified by the SVP manufacturer and are brand and model specific. One additional uncertainty factor has to be considered for the readability of the scale plate for open neck provers. This is estimated to be approximately one tenth of the smallest scale graduation. These are just typical numbers. In our experience of calibrating 27 small volume MEASURE | 71 REVIEW PAPERS provers (five runs each) and 11 open neck provers (ten runs each), our best standard deviation of the process was 10 ppm and the worst was 93 ppm with a median value of 22 for traditional open neck provers and 38 ppm for small volume provers. This variance is to be expected due to technique, drain characteristics, seal and valve integrity, and a multitude of other factors that make each standard a unique device. 15. C o n c l u s i o n s a n d L o o k i n g t o t h e F u t u re Based on this work, gravimetric water draw is a viable volume calibration option. It has the advantages of simplicity, realistic drain times, speed, and relatively low additional equipment outlay for the mass calibration laboratory. This is a new area for the State Weights and Measures laboratories. Work will continue in order to build additional long-term degrees of freedom into the process uncertainty analysis. A 100gallon control standard with a significant amount of volume transfer calibration history is being calibrated by gravimetric water draw now to compare the two methods directly. The calibration process will continue to be refined. Additional data will be collected and input from other laboratories will be solicited in order to continue to improve large volume calibration and subsequently, enhance meter proving capabilities in the United States. 16. A c kno wl e dg em e nts I thank the following: (1) The metrologists of the North Carolina Standards Laboratory, Bob Albright, Tal Anderson, Van Hyder, Cliff Murray, and Sharon Woodard who, in addition to working on the calibration procedure itself, made the hundreds of measurements referenced in this paper. (2) Henry Oppermann, Georgia Harris, and Val Miller of the NIST Weights and Measures Division, for providing support for travel and training, measurement evaluation, document review, and general wisdom. (3) Calibron Systems, Inc., Scottsdale, Arizona, for their technical expertise, SVP calibration spreadsheet that served as a starting point for ours, and for inviting several of our group visit their facil72 | MEASURE ity to observe their calibration of a SVP. (4) Kelleen Larson from the Arizona Department of Weights and Measures, and Craig VanBuren and Bill Erickson from the Michigan Department of Agriculture, E.C. Heffron Metrology Laboratory, for the hours of work developing, revising, and perfecting procedures. (5) The loss control specialists and SVP operators of Marathon Ashland Petroleum, LLC, for their incredible patience as we learned by experimenting with their SVPs. 17 . R e f e re n c e s [1] Energy Information Administration, Office of Oil and Gas, U.S. Department of Energy, Washington, DC 20585, Petroleum Marketing Monthly, March 2005. [Available from the web site eia.doe.gov/pub/oil_gas/petroleum/data _publications/petroleum_marketing_mo nthly/historical/2005/2005_03/pmm_2 005_03.html] [2] NIST Handbook 44, “Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices,” Table T.2, pp. 3-15, 2005 Edition. [Available from the website http://ts.nist.gov/ts/htdocs/230/235/pu bs.htm] [3] NIST Handbook 105-3, “Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures, Graduated Neck Type Volumetric Field Standards,” 1997 revision. [Available from the website http://ts.nist.gov/ ts/htdocs/230/235/pubs.htm] [4] American Petroleum Institute, “Manual of Petroleum Measurement Standards,” Chapter 4: Proving Systems, Section 7: Field Standard Test Measures, 2nd Edition, December 1998. [5] American Petroleum Institute, “Manual of Petroleum Measurement Standards,” Chapter 4: Proving Systems, Section 3: Small Volume Provers, July 1988. [6] American Petroleum Institute, “Manual of Petroleum Measurement Standards,” Chapter 4: Proving Systems, Section 6: Pulse Interpolation, November 2003. [7] J.K. Taylor and H.V. Oppermann, “Handbook for the Quality Assurance of Metrological Measurements,” NBS Handbook 145, pp. SOP 14-1 to SOP 14-4, 1986. [Note: Selected updates are contained in G. L. Harris, and J. A. [8] [9] [10] [11] [12] Torres, “Selected Laboratory and Measurement Practices and Procedures to Support Basic Mass Calibrations,” NIST IR 6969, March 01, 2003.] American Petroleum Institute, “Manual of Petroleum Measurement Standards,” Chapter 12: Calculation of Petroleum Quantities, Section 2: Calculation of Petroleum Quantities Using Dynamic Measurement Methods and Volumetric Correction Factors, Part 4: Calculation of Base Prover Volumes by Waterdraw Method, 1st Edition, December 1997, Reaffirmed March 2002. NIST Handbook 143, “State NIST Weights and Measures Laboratories Program Handbook,” Table 10, p. 66, March 2003. [Available from the website http://ts.nist.gov/ts/htdocs/230/ 235/pubs.htm] Gary Cohrs, “Water Density vs. Conductivity Using Archimedes’ Law for Density Measurement,” Handbook for the Quality Assurance of Metrological Measurements, Presentation for API 4.9.4 Committee Meeting, February 2004. J.B. Patterson and E.C. Morris, “Measurement of Absolute Water Density, 1 °C to 40 °C,” Metrologia, vol. 31, pp. 277-288, 1994. P. Giacomo, “Equation for the Determination of the Density of Moist Air,” Metrologia, vol. 18, pp. 33-40 (1982) and R.S. Davis, “Equation for the Determination of the Density of Moist Air,” Metrologia, vol. 29, pp. 67-70 (1992). www.ncsli.org 74 | MEASURE www.ncsli.org NEW PRODUCTS NEW PRODUCTS Sypris Test & Measurement, Inc. Adds New Calibrations Sypris Test & Measurement, Inc., a subsidiary of Sypris Solutions, Inc., has added Antenna, Isotropic/Electromagnetic Field Probe, Line Impedance Stabilization Network calibrations and RF Screen Room/Anechoic Chamber Certification to its suite of calibration service offerings. Current calibration services include AC/DC, RF, Physical Dimensional, and Temperature instruments and test equipment. For more information, 800-463-8786 or www.calibrationandrepair.com. Veriteq Announces Calibration Lab Monitoring System Veriteq Instruments, Inc. announces the release of viewLinc 3.0 enabling calibration labs to monitor critical applications and processes. Personnel can view and monitor temperature and RH environmental data locally or across an existing network with a standard web-browser. With multi-stage alarm notifications to cell phones, pagers and PCs, and simple installation, viewLinc 3.0 with Veriteq data recorders allows a faster response to fluctuating conditions before critical calibration processes are affected. viewLinc’s scalability enables viewing historical or real-time data from one to 100 loggers. Veriteq recorders store data internally, archive data automatically and provide temperature and RH accuracy to `/– 2 %RH and `/–0.15 °C, and A2LA accredited calibration. For more information, see www.veriteq.com. METRICA Opens Subsidiary in San Antonio, TX Metrica, S.A. de C.V., a company founded in 1992 and headquartered in Monterrey, Mexico, has recently opened a subsidiary in San Antonio, TX named Metrica Industries Co., which is designed to provide calibration services for industrial measuring instruments in the areas of mass, pressure and electrical. Metrica Industries Co. offers a wide variety of metrology services to support the ISO 9000 and TS 16949 Quality Standards, including: calibration of measuring instruments; fabrication of special measVol. 1 No. 4 • December 2006 uring devices used in the automotive industry; technical consulting; sale of industrial measuring instruments; equipment maintenance; training; analysis for measuring instruments; and, the most important of all, offering customized solutions for their clients. The main calibration laboratory is located in Monterrey, Nuevo Leon, Mexico with several mobile units designed to provide “on site” calibration services for each of their clients. For more information, please contact Mr. Moisés Rivera at moises@metrica.com.mx or Mr. Roberto Benitez at roberto@metrica.com.mx. ACLASS Signs APLAC MRA At the 12th Annual General Assembly in Taipei, Chinese Taipei, ACLASS signed the Asia Pacific Laboratory Accreditation Cooperation (APLAC) Mutual Recognition Arrangement (MRA) on September 13, 2006 for both calibration and testing. APLAC groups accreditation bodies in the Asia Pacific region that are responsible for accrediting calibration, testing and inspection facilities. As a result of signing the APLAC MRA, ACLASS, already an associate member in ILAC, now automatically progresses to full membership of ILAC, in the fields of testing and calibration. A formal signing ceremony for the ILAC MRA will take place November 12, 2006 in Cancun, Mexico. ACLASS is headquartered in the Washington, DC metro area and is an accreditation body for the ISO/IEC 17025 standard. For more information, www.aclasscorp.com. A2LA Adds Inspection Body Accreditation to its APLAC Scope of Recognition A2LA has successfully completed the Asia Pacific Laboratory Accreditation Cooperation (APLAC) peer re-evaluation process. A2LA recognition for testing and calibration has been extended for four years. In addition, this year, inspection body accreditation to ISO/IEC 17020 and ILAC/IAF A4 has been added to its scope of recognition. “A2LA’s addition of inspection body accreditation to our MRA signatory status gives the program credibility and instant worldwide recognition,” said Peter Unger, President of A2LA. “A2LA accredited inspection bodies can now benefit from the same international recognition and credibility that our testing and calibration laboratories have enjoyed for years.” For more information, www.A2LA.org. DeFelsko Redesigns PosiTector 200-Advanced Coating Thickness Gage The PosiTector 200 coating thickness gage has just been redesigned to include new features, new measurement ranges and new models. The updated PosiTector 200 is ideal for measuring the total thickness of a coating system or up to 3 individual layer thicknesses in a multi-layer system. New features include extended range probes, faster measuring speed, large impact-resistant Lexan® display, IP5X ingress protection and protective rubber holster. Conforms to ASTM D6132 and ISO 2808. For more information, (800) 448-3835 or www.defelsko.com. Fluke Offers 8845A/8846A Precision Digital Multimeters The new Fluke 8845A and 8846A Precision Digital Multimeters feature 6.5 digit resolution, have a dual display that shows data in graphic or numeric formats, and provide multifunction measurement capability. They have 14 measurement functions, extending the capability of a standard DMM with wider ranges and features to measure temperature, capacitance, period and frequency. The 2 x 4 ohms function uses patented split terminal jacks that allow users to perform 4wire measurements using only two leads instead of four. The meters measure volts dc with an accuracy of up to 0.0024 %, have a voltage range of 100 mV to 1000 V with up to 100 nV resolution, current range of 100 µA to 10 A with up to 100 pA resolution, and have a wide ohms measurement range from 10 Q to 1 GQ with up to 10 µQ resolution. For more information, (888) 308-5277, or www.fluke.com/884XA. Continued on page 77 MEASURE | 75 76 | MEASURE www.ncsli.org NEW PRODUCTS Guildline Introduces New Series Of DCC bridges Hart Scientific Announces Temperature/Humidity Logger Guildline Instruments Ltd. recently launched the 6622A Series of Direct Current Comparator Bridges for resistance and thermometry applications. There are five models in the 6622A series, each with different measurement range and accuracy. Best uncertainty is 0.04 ppm and the widest range from 1mQ to 1GQ. These bridges are fully upgradeable in measurement uncertainties and ranges. A measurement ratio of 100:1 is available on all models, contributing to lower uncertainties when laddering up/down, reducing the number of standards a lab needs and making it possible to verify the bridge performance without a Hamon style transfer standard. Optional range extenders can expand the measurement range down to 0.1 µQ at up to 4200A maximum current. Hart Scientific has announced the new Model 1620A, “DewK,” paperless temperature and humidity data logger with wireless, Ethernet, and RS-232 communications. Designed to facilitate the electronic management of environmental temperature and humidity data, this thermo-hygrometer is intended for critical locations such as calibration and research labs, pharmaceutical and chemical storage areas, and many medical environments. The DewK accepts inputs from up to two sensors, which may be mounted directly on the unit or up to 30 meters away. Two sensor models are available. For more information, www.guildline.com. For more information, www.hartscientific.com. Integrated Sciences Group Metrology Handbook The Analytical Metrology Handbook, introduced at the 2006 NCSLI Workshop & Symposium, is available for $25 from Integrated Sciences Group at isgmax.com. Topics include measurement uncertainty analysis, calibration interval analysis, measurement decision risk analysis and SPC for measurement processes. Order on-line, or call 1-800-400-7866. Keithley Metrology Achieves ISO 17025 Accreditation Keithley Instruments, Inc. announced that its Metrology Services Group has been accredited to ISO/IEC 17025:2005 by the American Association for Laboratory Accreditation (A2LA). This accreditation recognizes that Keithley’s Metrology Services meet the requirements of this international standard, demonstrating its technical competence to carry out very high-level calibrations that are essential for many of Keithley’s instruments, which have Continued on page 78 Vol. 1 No. 4 • December 2006 MEASURE | 77 NEW PRODUCTS measurement resolutions below 1 femtoamp. The scope of accreditation can be viewed at www.a2la.org/scopepdf/ 2462-01.pdf. Keithley’s customers can now obtain accredited calibrations for Series 2300 sources, Series 2600 and Series 2400 SourceMeter® instruments, Integra Series products, Series 2000 DMMs, and the Model 6517A Electrometer. For more information,www.keithley.com/pr/057. Mettler Toledo Introduces Two New Comparators In metrology, comparators are used to determine the mass of weights or samples to the highest degree of accuracy. The new XP56C Comparator from Mettler Toledo has a continuous weighing range of 52 g and a readability of 1 µg (1/1000000 g), In combination with its dedicated hanging weighing pan that eliminates corner load errors, the XP56C Comparator guarantees the world’s most accurate and repeatable results. The 78 | MEASURE WeighCom application guides the user step by step through the mass determination process, and SmartScreen, the color touchscreen display, enables convenient and error-free operation. by Precision Measurements. Using state of the art automated calibration equipment, Precision Measurements can also repair and calibrate all thermal converters, independent of their manufacturer. For more information, Pascal.Desponds@mt.com. For more information, www.measure-tech.com. Precision Measurements AC/DC Transfer Standard A new process for manufacturing thermal converters has been developed by Precision Measurements. Each unit is now being manufactured with Evanohms’ heater wire and cold bead, which results in lower AC/DC and reversal errors. This unit has been tested by national labs and proven to have the lowest errors. The versatility of the new process allows the use of platinum leads and Evanohm leads for the heater wire, which results in extremely flat response to 100 MHz and beyond. Both the new vacuum thermocouple and the thermal converter are manufactured Stranaska Develops UV/VIS Calibration Artifacts Holmium oxide solutions, comprised of 4 % (w/v) holmium oxide in aqueous 10 % (v/v) perchloric acid, provide traceability for the calibration of absorption spectrophotometer wavelength scales within the spectral range of 240 nm to 650 nm. To mitigate extrapolated wavelength calibrations outside this traditional wavelength range, Stranaska has developed two different series of wavelength standard artifacts which facilitate science-based measurement traceability for reference wavelength assignments above 650 nm and below 240 nm. One series is comprised of new calibration artifacts certified for wavelength assignments in the spectral range 200 nm to 240 nm, and a second series is www.ncsli.org NEW PRODUCTS certified for wavelength assignments in the spectral range 650 nm to 900 nm. Tegam Introduces RF Power Calibration System IIB Tovey Engineering Offers Torque Calibration Systems For more information, 970-282-3840, www.stranaska.com. The new Tegam System IIB is an updated version of the Tegam/ Weinschel System IIA. The new System IIB offers better functionality, fewer components and a lower price. The central component of the System IIB is the new 1806A Dual Type IV Power Meter, which allows for more precise RF power control when calibrating higher frequency, multi-range, or high and low power sensors. Tegam offers a SYSIIB Selection Guide online to assist with package selection based on power sensor calibration needs. Tovey Engineering transfer standard torque calibration systems are now available in capacities ranging from 200 in-lb to 100,000 in-lb. These fully automated systems permit efficient in-house transducer calibration. Systems include: torque frame, transfer standard load cells, hydraulic actuator, and software for automated control and data analysis. The systems feature full automation, precision machining, very high accuracy reference standard load cells, reduced misalignment errors, efficient transition from clockwise to counterclockwise calibration, and true zero deadband measurement. Team Torque Inc. Announces A2LA Accreditation Team Torque Inc. announced the latest accreditation of their Calibration Laboratory and Customer Support Center. The American Association for Laboratory Accreditation (A2LA) awarded the company with accreditations to ISO 17025 and Z540-1, the top international standards for calibration labs. Team Torque Inc. provides calibration and repair for all torque tools and calibration test equipment. This recognition is designed to prove that these services are conducted at the highest level of technical competency available in the industry. For more information, contact Jim Mueller at (701) 223-4552, ext. 21. For more information, call Kim Niznik Goff, at (440) 466-6100, visit www.tegam.com. Product information is provided as a reader service and does not constitute endorsement by NCSLI. Contact information is provided for each product so that readers may make direct inquiries. For more information, (623) 434-5110 or see www.toveyengineering.com. ADVERTISER INDEX A g i l e n t Te c h n o l o g i e s www.agilent.com.................................................... 18 T h e A m e r ic a n A s s o c ia t i o n f o r L a b o r a t o r y A c c re d it a t io n I n t e g ra t e d S c i e n c e s G ro u p www.isgmax.com ........................................ 20 L a b o r a t o r y A c c re d i t a t i o n B u re a u www.l-a-b.com.................................. 74 ( A 2L A ) www.a2la.org ............................................................................ 9 L e a r n i n g M e a s u re www.learningmeasure.com ........................................ 80 A nd e e n - H a g e rl i n g , I n c. www.andeen-hagerling.com .............................. 76 Ma s y S y s te ms , I nc . www.masy.com ........................................................ 11 A s s e t S m a r t www.assetsmart.com ............................................................ 31 M e n s o r C o r p o r a t i o n www.mensor.com .................................................... 4 B l u e M o u n t a i n Q u a l i ty R es o u rce s www.coolblue.com .......................... 16 M o reh o u s e I n s t r u m e nt C o . www.mhforce.com ...................................... 30 C al La b www.callabmag.com .................................................................... 80 N o r t h ro p G r u m m a n C o r p o r a t i o n www.northropgrumman.com ............ 13 D a t a P roo f www.dataproof.com ................................................................ 76 Oh m - L a b s www.ohm-labs.com ................................................................ 76 D H I n s t r u m e nt s , I n c . www.dhinstruments.com ...................................... 6 D H - B u d en b e rg I n c . www.dh-budenberginc.com .................................... 21 Q u a m e t e c C o r p o r a t i o n www.quametec.com ........................................ 21 D y n a m i c Te c h n o l o g y, I nc . www.dynamictechnology.com ...................... 10 S p ek t r a www.spektra-usa.com.................................................................. 80 E d i s o n M e t ro l o g y www.edisonmetrology.com ........................................ 73 Sy pr i s Te s t an d M e as u re m e n t www.calibration.com .............................. 14 E s s c o C a l i b r a t i o n www.esscolab.com .................................................... 77 TA C / To u r A n d o v e r C o n t ro l s www.tac.com/pe ........................................ 78 F l u ke / H a r t S c ie n t if ic C o r p o r a ti o n www.fluke.com ........................................................ T h u n d e r S c i e n t if i c Co r p o r a ti o n Inside Front Cover, 11 G u lf Ca li b ra t io n S y st e m s www.gcscalibration.com .......................................... Outside Back Cover H e u s s e r N e w e i g h www.neweigh.com ...................................................... 74 H o l t I n s t r u m e n t www.holtinstrument.com ................................................ 76 Vol. 1 No. 4 • December 2006 P ro c e s s I n s tr u m e n t s I n c. www.procinst.com .......................................... 45 www.thunderscientific.com ............................................Inside Back Cover To v e y E n g i n e e r i n g www.toveyengineering.com ...................................... 17 Vai s a l a I nc . www.vaisala.com.................................................................... 45 Ve r i t e q www.veriteq.com .......................................................................... 15 MEASURE | 79 CLASSIFIEDS NCSLI Training Center The NCSLI Training Center is designed to provide a state-of-the-art training facility with full electronic and staff support. NCSLI member organizations are charged a reduced rate. 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