Marek Bartkowiak - The International Society for Sample Environment

TEMPERATURE
SENSORS
Marek Bartkowiak
Overview
• ITS-90
• low temperature extension
• common sensors
• range
• thermocouples
• PTC resistance thermometer (PT RhFe)
• NTC resistance thermometer Cernox RuOx
• Diodes
• sensor calibration
• special sensors - examples
practical approach
temperature is a thermodynamic property of state
it can be defined with a reversible cycle (Carnot)
this is NOT practical
better:
when 2 bodies in thermal equilibrium that are brought
together do not exchange energy they are at the same
temperature
==> bring some material in contact with the specimen and
measure its temperature
To measure temperature:
use any physical property that varies monotonically in the
T-range of interest
color of a
glowing object
pressure of
a fixed gas volume
vapour pressure
volume
phase transitions
magnetisation
dielectric
properties
electromagnetic noise
thermo-electric
effect
resistivity
ITS-90
(International Temperature Scale)
based on fixed points
(freezing points, triple
points) and
Helium gas and vapourpressure thermometry
lowT extension: PLTS-2000
based on 3He meting curve
TN TB TA
TMIN
Temperature scale
to transfer a temperature scale one can use
*primary thermometers: devices whose temperature dependent
physical property can be obtained from principle laws of physics
examples: Gas and vapour pressure thermometer, noise thermometer,
coulomb blockade, nuclear orientation thermometer
-difficulty: the entire measurement set-up need to be understood
practically a reference point which was calibrated against the
ITS is used for the transfer (triple points, superconducting
transitions...)
*secondary thermometer: not all details of the temperature dependence
of a property are known. individual sensors need to be calibrated in the
entire temperature range
these thermometers are used in experiments where temperature is a parameter
magnetisation
dielectric
properties
color of a
glowing object
pressure of
a fixed gas volume
vapour pressure
electromagnetic noise
phase transitions
volume
thermo-electric
effect
resistivity
Sensor selection
• define the temperature range to be covered
• sensitivity requirements
• response time
• mounting possibilities
• magnetic field sensitivity
• radiation resistance
Sensitivity
T
Normalized Sensitivity
for resistive
thermometers
Slope scales with
absolute resistance
value
==> a relative slope
is gives a better
comparison
Dimensionless Sensitivity
logSignal
Signal
Sensitivity
logT
best to compare sensitivity
over large range
Resistance
Sensitivity
Example
normalized Sensitivity
dimensionless Sensitivity
Accuracy
accuracy is combination of sensor sensitivity and
measurement accuracy of the Temperature
controller
for a Resistance measurement with accuracy ΔR:
Thermocouples
• based on thermoelectric effect between dissimilar metals
• very local probe fast response
• simple
• no intrinsic heating
• good reproducibility
• usually insensitive to magnetic field
• cons: sensitivity drops to 0 at T=0
• not very accurate
• practical limit >10K
• grounding issues
https://www.picotech.com/library/application-note/thermocouple-application-note
thermocouples measure a temperature
difference
Chromel
Iron
Copper
0
A current flows in a loop of dissimilar
metals when the joints are kept at
different temperature (Seebeck-Effect)
V
“thermo voltage” with Pt reference
Temperature
Alumel
Constantan
Type K (chromel/alumel) is mostly used ==> huge variety of form factors
Thermoelectric voltage
• tabulated values are always given with
respect to a reference temperature
(T2=0°C)
●
reference has to be very stable (ice
water) or corrections have to be made
T1
T3
T2
T3
T1
hot spots along the path of the
thermocouple should not
influence the measurement,
but they do in practice due to
the extend of these hot zones
and transient effects
T2
intermediate metals will not
influence measurement when
kept at a single temperature
Resistance Thermometry
huge variety of sensors but 2 groups:
PTC: positive temperature coefficient
(metals)
NTC: negative temperature coefficient
(semiconductors)
problematic: •
•
•
•
electromagnetic noise
thermal anchoring
selfheating
sensitivity to magnetic
fields (magneto
resistance)
General Construction
simple thermal model
response time:
Overheating (Joule Heating)
Diodes: U increases with
decreasing temperature
==>I.. needs to be small
RTC: R decreases with
decreasing temperature
==>constant current controller
NTC: R increases with
decreasing temperature
==>constant voltage controller
to test if overheating is occurring measure at stable condition with various
excitations
Metal Resistance Thermometer
• Most typical material Platinum (large temperature
•
•
•
•
coefficient) PT-100 PT-1000
follows a standard calibration
different grades of accuracy +cheap
very good between 77K and 800K
fair to 20K
Rhodium-Iron
• covers a large temperature
range (1.4K-800K)
• good sensitivity to 50K
• fair below
• expensive
Cernox Sensors
•
•
•
•
thin-film ceramic Zirconium Oxynitride sensors
commercially produced
film thickness 0.3μm on sapphire substrate
temperature characteristics can be tuned during the
sputtering process
• cover a large temperature range between 0.1K to 400K
• different packaging available
• insensitive to magnetic fields above 2K
Ruthenium Oxide
• thick-film resistors based on RuO2 metal ceramic composite
• various suppliers (Lakeshore, Scientific Instruments,OI, Entropy, Vishay)
• good reproducibility (fair agreement with standard curve)
• weak magneto-resistance
• most sensitive below 10K
in a limited T-range:
Si-Diodes
T1 > T 2 > T 3
Current
• based on the forward voltage drop at p-n transition
• signal increases with decreasing temperature
==> selfheating limited
• constant sensitivity between 10K and RT
• follows standard curve
• fast response
Voltage
Radiation hardness
Si-diodes are not recommended
for use in X-ray or Neutron
radiation
Compiled data from Lakeshore
Sensor mounting
use preferably a dead end geometry
Sample
Thermometer
Sample holder
Heater
Thermal link
Thermal bath
(cooler)
preferably designed as a
weakly conducting link
Sensor calibration
...is a business of trust.
trust lent to the calibration of a manufacturer
(can be improved by having different sensors from different sources)
and trust in the home made equipment (and devices used for calibration)
(I.e. pressure sensors, resistance bridges,...)
little trust in others and high level of confidence: -very expensive, labour intensive
-need to understand and control
the entire calibration process
-have suitable references
available
some trust in others : -make cross checks of different sensors
-be aware of faulty readings, recheck calibrations
of sensors and measurement devices
fully trust others:
-sample temperature is the number on the display
-costs: your reputation and creditability
Home brew thermometers
●
for ULT setups not many sensors are available
==> constructing your own packaging might be necessary
calibration set up
HALL-9500 µSR spectrometer
Sensor with thermal anchor
and HF-filter
weak link
(brass, stainless)
CMN
SR1000
fix point device
Sensors
Home brew thermometers
Motivation:
thermal connection between sample and mixing chamber via 10mm Cu rods
to adapt the length to a specific cryostat or magnet
thermometer should be close to sample, easy to mount not take a lot of extra
space (max diameter <16mm)
weak link extends the
accessible temperature
range at the expense of a
higher base temperature
w/o WL: 0.05-0.8K
w WL: 0.1-4K
Home brew thermometers
Sensor and heater for Kelvinox
field insensitive RuOx (EntropyC)
dual use as heater
combi sensor
Cernox and RuOx
Literature
• Pobell, Matter and Methods at Low Temperature, Springer (2007)
G. Ventura & L. Risegari The Art of Cryogenics, Elsevier New York (2007)
• http://www.lakeshore.com
• G.Schuster, Rep. Prog. Phys. 57 (1994), 187-230
• http://www.scientificinstruments.com
https://www.picotech.com/library/application-note/thermocouple-application-note