How to Guarantee Your Failure as an Infrared Thermographer Ronald Lucier

How to Guarantee Your Failure as an Infrared Thermographer
‫چگونه عیوب حرارتی شناسایی شده توسط دوربین ترموگرافی را به عنوان یک ترموگرافر حرفه ای تضمین کنیم ؟‬
Ronald Lucier
ABSTRACT
The usefulness of thermal imaging in Predictive/Diagnostic Maintenance (PDM) has been demonstrated and
documented for over twenty five years. Practitioners in the trade generally fall into two categories – heroes or
goats! This paper highlights ten common activities that if not properly addressed can lead a thermographer down
the path to failure. By recognizing and avoiding these pitfalls the paths to success will be much clearer.
‫ فعاالن تااری در این زمینه االاا به دو‬. ‫ سال می باشد‬52 ‫کاربرد ترموگرافی در تعمیرات پیشگیرانه یا پیش بینانه دارای سابقه ای بالغ بر بیش از‬
‫ عنوان مهم در بازرسی ترموگرافی می پردازد که در صورت عدم‬01 ‫ این مقاله به برجسته سازی‬. ‫دسته قهرمان و بازنده تقسیم بندی می شوند‬
‫ با رعایت این اصول بنیادی درصد موفقیت و تشخیص دقیق عیوب افزایش می یابد‬. ‫توجه به آنها باعث بروز اشتااه در تشخیص اپراتور می گردد‬
.
Keywords: Quantitative thermography, qualitative thermography, corrective action, risk, image
subtraction, IRNDT
‫ تست غیر مخرب ترموگرافی‬، ‫ تحلیل تصاویر مادون قرمز‬، ‫ خطر‬، ‫ اقدامات اصالحی‬، ‫ ترموگرافی کیفی‬، ‫ ترموگرافی کمی‬: ‫کلمات کلیدی‬
THE DISTANCE BETWEEN SUCCESS AND FAILURE CAN BE A FEW MICRONS
The successful track record of infrared thermography as a predictive maintenance tool is well established. What
isn’t so well established are programs and efforts that have not been successful. Over nearly twenty years of
experience I have seen both the successes and failures. The failures seem to have many things in common. In fact
there are ten common attributes. Identifying and avoiding these ten attributes may not guarantee your success.
Ignoring or not recognizing some of these attributes however will probably guarantee your failure.
TEN CRITICAL PITFALLS
1. Over Emphasizing Temperature Measurement
Ever since the introduction of radiometric systems there has been a tendency to rely on temperature measurements
as an indicator of component operability. NETA1, and others have generated “equipment severity ratings” based on
temperature. Therefore, through training and subsequent research, infrared thermographers have become
accustomed to providing temperature measurements in their reports.
There should be several key questions you ask yourself about temperature measurements:
!
What is the relative accuracy of the measurement?
!
Could I defend this measurement in court, against an Expert Witness?
!
Does the temperature measurement provide meaningful information?
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It is important to note that Infrared Cameras do not measure temperature. Radiant energy is measured (Watts) and
from that value and your inputs temperature is calculated. Your “measurement” accuracy is directly related to the
accuracy of the camera calibration and the accuracy of your inputs for emissivity, background temperature and any
applicable environmental parameters. This also assumes you have the skill to acquire an image in focus and within
the spot size ratio of your system.
Potentially there could be liability for your measurement. Trial lawyers search wide and search deep for anything
that could help or hurt their client’s case. If you report a temperature, particularly for a client other than your
employer, be sure of your results and express the uncertainties in your measurement.
A larger issue for “temperature measurement” is whether that measurement is meaningful or not. The two
thermograms below show a battery and cables. There is a 9 ºF difference between the good battery stud and the one
overheating. The absolute temperature of the bad stud is 119 ºF, +/- 3ºF. The melt point for Lead is 620 ºF.
Therefore under most common severity guidelines this would be classified as a MINOR problem.
The important information regarding this component is not necessarily the temperature but the electric current (load)
running through it. The stud is hot due to high resistance. As this is a DC application, the power dissipated at the
connection can be calculated using the equation P=I2R. At the time of measurement the battery was being trickle
charged at 60 ma (0.06 Amps). The battery was rated for 200 amps. Ignoring the fact that the resistance and heat
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transfer will increase with temperature, a simple ratio of the power dissipated, at a load of 200 Amps the connection
would dissipate about 11 Million times the power it is in the image. Obviously, this would cause a failure.
Thermograms of battery and cables
Lesson Learned
The important point here is that this is one example where the actual temperature measurement or temperature rise
measurement is nearly meaningless unless the current is also noted. Reporting just the temperature and acting based
on that temperature could have resulted in a battery fire. The load (current) is crucial in electrical system analyses.
2.
Ignoring Temperature Measurement
OK, so there could be big problems with temperature measurements. Maybe it is best to do everything qualitative?
Let’s look at one example.
108.2°C
The thermal image of a breaker with a hot conductor to the right is a
classic example of where qualitative thermography can lead to errors.
As the conductor temperature is highest at the connection point, the
assumption of a loose/corroded connection may be a good one.
Therefore the typical PDM report may state that the corrective action is
to clean and tighten the connection. In fact this may solve the
connection problem but is the connection problem the whole story? In
this case a secondary problem arises from the fact that the conductor
temperature is 115 ºC, at or above the temperature limit on the cable
insulation. This would result in the corrective action of
cleaning/tightening in identifying the more serious issue – the insulation
has degraded to the point where new conductor must be pulled to correct the problem. In this real life example the
insulation was hard and broke off when the “connection” problem was to be fixed. A quick repair turned into an
major effort and considerable equipment down time.
Lesson Learned
Quantitative thermography is essential when equipment temperature limits are approached. There are many other
examples where quantitative measurements are required such as diagnosing steam traps, safety relief valves or
determining corrective action priority. The important point here is BALANCE – look at what you are looking at,
how important it is and based on that knowledge determine whether qualitative or quantitative thermography is
necessary. This is not as obvious as some may think.
When you are unsure, get help. Get the entire PDM team including engineering, operations, management and
maintenance involved. A wrong decision by a group is “a learning experience.” A wrong decision by an individual
is a “bad call.”
3. Crying Wolf!
It should be obvious that extreme care must be taken when determining corrective action priority. It should also be
apparent that temperature alone cannot always be the final decision maker. The danger is that unnecessary repairs or
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100
80
60
40
25.8°C
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shutdowns will be asked for or that no action is called for and the component fails. Either way, the infrared
thermographer becomes the goat and most companies sacrifice goats!
ITC is aware of nearly a dozen corrective action guidelines, almost all relating to temperature. None of these are
universally applied to mechanical and electrical equipment. In fact as demonstrated above a temperature based
corrective action guideline can easily yield undesired results.
A concept that can be universally applied to all corrective action guidelines is to assess the “Risk of Failure” for an
individual component.
The definition of risk is:
RISK = (probability of failure) x (consequences)
The problem here is that every equation requires finite numbers to plug in to that equation. Unfortunately neither
the probabilities nor the consequences are easily quantified.
Probability of failure
In mathematical terms the entire range of probability ranges between zero (will not happen) and one (will happen).
To quantify this number for every application would require a statistical analysis of hundreds of thousands of
failures under known conditions. However, using your own temperature data, prior equipment history and perhaps
intuition, the probability of failure can be broken into LOW, MEDIUM and HIGH categories. Likewise the
consequences. What you end up with can be placed in a matrix.
PROBABILITY OF FAILURE
Based on:
Temperature
Temperature Rise
Past History
Industry Experience
Intuition
CONSEQUENCES
Based on:
Equipment Damage
Fire Potential
Personnel Injury
System Shutdown
Availability of Parts
Scheduling
Lesson Learned
PROBABILITY OF FAILURE
C
LOW
O
N
LOW
Q
MEDIUM
U
HIGH
NORMAL
ACCELERATED
REPAIR NEXT
REPAIR
REPAIR
EQUIPMENT
SCHEDULE
OUTAGE
ACCELERATED
REPAIR NEXT
REPAIR AS
REPAIR
EQUIPMENT
SOON AS
SCHEDULE
OUTAGE
POSSIBLE
REPAIR NEXT
REPAIR AS
REMOVE FROM
EQUIPMENT
SOON AS
SERVICE
OUTAGE
POSSIBLE
ASAP
S
E
MEDIUM
E
N
C
E
HIGH
S
In a perfect world action criteria
would be based on hard science rather than a subjective determination. However, that is not the case, but by
utilizing your own experience, knowledge and intuition, a better method of corrective action priority should yield
better results.
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4.
Misidentification of Components
The many hats that infrared thermographers have to wear includes the hat that says “EXPERT”. Whether you like
it or not you are considered the expert in whatever you are looking at with your IR camera. Whether you really are
or not may be discovered by reviewing your reports. Choose from the two below. Which thermographer do you
think needs some education?
49.4° C
49.4° C
48
48
46
46
44
44
42
42
40
40
38.6° C
38.6° C
IR Text Comment
Value
IR Text Comment
Value
Equipment
Black Box
Equipment
Breaker for P-38 Pump
Item
Wire on bottom
Item
Load side conductor, B Phase
Fault
Wicked Hot
Fault
Lug is 7 ºC hotter than adjacent lugs
Recommendation
Fix It
Recommendation
Inspect for damage, clean, tighten
Re-inspect after repair
It is very tough for a non electrical person to identify electrical components. There is no excuse for not correctly
identifying components yet this is done every day.
Sources for the correct identification of components include plant personnel such as mechanics, electricians,
technicians as well as engineers, management, equipment operation manuals and the industry groups on the internet.
Lesson Learned
You should be able to produce a professional looking report with nice visual and infrared pictures. You should also
correctly identify the components you inspected. If you don’t know what it is called, find out!
5.
Not Having an Open Mind
I have been promoting the concept of IRBWA (Infrared By Walking Around) for many years. What this means is
that while you have a thermal imager in your hands, take time to look at as much as you can. We live in a thermal
world and there are all sorts of interesting things out there, some of them important! Much of what you see may not
have much relevance. Some of it will. I was with an electrician in a manufacturing plant not too long ago when he
took the IRBWA concept to heart. He located and reported a bad bearing on a conveyor belt roller. A simple task to
many of us but to this electrician it opened up a whole new world to him. Some of the images you obtain with
IRBWA can be very interesting. Some even useful!
There is a downside to not practicing IRBWA. Everyday that you use an Infrared Camera you should be increasing
your own knowledge base. A narrow or limited knowledge base limits your experience and may limit your future in
infrared thermography.
Lesson Learned
Unless your work rules prevent it, keep the camera running! You never know what,s lurking out there.
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6.
Not Understanding the Science
It has already been noted that Infrared Cameras do not measure temperature. There are organizations that know this
but make temperature measurements with a fixed value of emissivity. If your program is set up to fix everything
that is found, regardless of temperature or temperature rise, this is ok. If your program is set up to react based on a
temperature or temperature rise you should know the science and get the temperatures as accurately as possible.
The value that the infrared camera uses to calculate the temperature is the radiated energy from the target surface.
Once that is known, a simple Physics model based on the Stefan-Boltzman Law ( W = !"T 4) is used to determine
the temperature. Before the calculation occurs, several corrections are made.
Detected Radiance minus measured internal camera equivalent temperature (due to electronics, case, lens)
Plus
Energy lost through the lens (transmittance of the optical materials)
Plus
Energy lost through the atmosphere (distance, humidity, atmosphere temperature)
Minus (1-emissivity)(Tbackground temperature) this is Tamb in our systems and is the reflected temperature
Finally divide by the emissivity. This is the value used in the calculation above to calculate temperature
The net result of this is that if you are expecting a correct measurement you must put in correct values for emissivity
and Tamb. The distance, humidity and atmosphere temperature have a larger contribution in shortwave (3 to 5 µm)
than longwave (8 – 12 µm) systems. Putting the wrong values in every time will yield the wrong measurement out
every time!
Lesson Learned
Understand the science and you will be able to defend your measurements or your decision to not make a
measurement.
7.
Not Understanding IR Limitations
Beyond misidentification and errors in emissivity, the most common problem we see in infrared reports is being too
far away to make a measurement. The infrared camera utilizes the detected radiance values over a certain number of
pixels to calculate the temperature. It is the job of the camera operator to make sure the target is physically or
optically close enough to make this measurement. In other words, know your Mfov (measurement field of view) and
Spot Size Ratio (1/ Mfov )
An additional concern is low emissive targets. Whenever possible, particularly when electrical equipment is deenergized, apply a high emissive material such as opaque electrical tape to the equipment. This will give you a
highly emissive target (the tape) at a known emissivity from which to make your measurements.
Lesson Learned
There is a huge abundance of technical information provided by IRTECH, ITC and others. This material is
worthless if it stays on a shelf and collects dust. Periodically review any relevant technical information and
specifically review your camera operating manual (or read it for the first time…).
8.
Not Understanding IR Capabilities
One of the least utilized capabilities of Infrared Cameras is the ability to analyze objects on a time dependent basis.
This can be accomplished by recording live events (starting a component, filling a tank, mechanically loading a
structure, etc) or taking a series of pictures and applying the techniques of image subtraction.
Analyzing live or recorded events allows you to plot temperatures as fast as the infrared camera can store them.
This can be as fast as 900 frames per second with the FLIR SC-3000, up to 30 frames per second with the SC-2000
series (includes 695 with the digital board) and a frame grabber, up to 5 frames per second through the PCMCIA
card on a lap top computer or up to as many seconds, minutes, hours as you need. Geometric registration (fixed
camera viewpoint) is essential.
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The speed that a process needs depends largely on the temperature difference, the amount of heat flowing through
the object and a heat transfer property called “thermal diffusivity”, which is basically how fast heat conducts
through a material. All of this forms the basis for Infrared Non-Destructive Testing (IRNDT). In the IRNDT case
the basic principle is to flow a uniform amount of heat into a surface and see what happens to it. If it appears
uniform out of that surface then the material is uniform inside. If not, a non-uniformity may exist due to a variety of
reasons (de-lamination, voids, inclusions, etc).
Image subtraction is the process where two thermograms, geometrically registered, are obtained and one is
subtracted from the other. This process results in a “difference thermogram”, showing how the individual pixels
changed in temperature. This is commonly used to analyze changes on the surfaces of semiconductors and/or
printed circuit cards.
Start of sequence
End of sequence
23.4dF
23
13
22
11
21
9
7
S28
20
34
45
23
1
12
5
19
S1
18
18.0dF
Difference Thermogram
Surface Plot of Difference
The images above are from the warm side of a simple Peltier Cooler. The first image is one second after the
application of power. The second image is ten seconds later. Obviously the amount of heat removed from the
opposite surface is evident. The third image is the difference thermogram – Image one minus Image two. What was
not apparent from the first image is that there are several relatively hot pixels. In fact a Peltier Cooler should be
expected to be fairly uniform in temperature. This one was not. Image subtraction provided the answer!
The difference thermogram and the Excel surface plot show the non-uniformity that is hard to see in the end of
sequence image. What is also apparent is that this technique subtracted out the background effects – only the Peltier
cooler is visible.
Lesson Learned
There are other capabilities, particularly spectral filtering, that are under utilized. Keep and Open Mind about what
your camera can do!
9.
Over Reaching
An infrared camera has more uses in industry and science than probably any other device yet invented.
Unfortunately its proper operation is bounded by the Laws of Physics. The Infrared Thermographer must balance
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his or her enthusiasm (and perhaps the need to generate an invoice) by these Laws. Don’t over reach your (or the
camera’s) capabilities.
One example of “over reaching” is the attempt to locate a ghost. The Learning Channel and other television shows
have hunted for ghosts with psychics, acoustic instruments, ultrasonic instruments and infrared cameras. The results
have been very conclusive – infrared cameras do not work for this application. Again, it is those darn Laws of
Physics. Infrared cameras detect energy emitted from solid surfaces. By definition a ghost has no solid surface so
how could an infrared camera detect them? One television show indicated that a ghost must have been present as a
change in room air was detected. Next time you are in an air conditioned hotel room, look for the cold plume of air
coming out of the air conditioner. Or that draft in your house. You see the walls, ceiling and floor thermal patterns
but not the air as the solid surfaces are very small and very far apart (definition of a gas).
Perhaps if we could utilize a spectro-radiometer and determine the spectral characteristics of the specter (snicker)
we could design a filter…. Better yet, stick to looking for poor insulation in houses.
Lesson Learned
There are ample uses for infrared cameras beyond routine PDM applications.
10.
Being Complacent
All right, you have many years of experience, have been to many training courses and wonderful conferences like
InfraMation and you regularly interface with your peers. You must “Know It All”. Think again.
Ask the most experienced person you meet at this conference if they “Know It All”. They will answer NO. Infrared
Thermography is a science with many, many applications. It is nearly impossible to know and be competent in all
aspects of this science. Even if you stick with one discipline, do not be complacent. There is still much to be
learned about electrical, mechanical and roofing applications. If there wasn’t there would be no point in attending
InfraMation!
Lesson Learned
Never stop learning and never take what you see for granted. Catastrophic equipment failures can and will occur
despite your best efforts to identify them beforehand.
SUMMARY
This paper describes ten things that can lead you down the path to failure. Obviously we do not want anyone to fail
but to rather be an enormous success. While there may be more than these ten paths to failure there are only three
paths to success:
People
Hire the best, train with the best and interface with the best. Remember, attending a training course or two is not the
end of your education but only the beginning.
Resources
Know what resources are available to you and utilize those resources. Keep in touch with your infrared camera
manufacturer, trainer, customers, co-workers and frequently review any available technical documentation.
Technology
Acquire the best technology you can and make sure you understand how to operate it!
REFERENCES
1.
ANSI/MTS-2001, “Maintenance Testing Specifications for Electric Power Distribution Systems”, NETA
InterNational Electrical Testing Association, (2001)
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