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Self-Validating Thermocouples For in-situ calibration drift correction
Claire J Elliott, Jonathan V Pearce, Graham Machin
ESA-estec: Christian Schwarz, Robert Lindner
6 November 2012
The UK’s national standards laboratory
Develop & disseminate UK’s measurement
standards, and ensure they are
internationally accepted
Founded in 1900
500+ specialists in Measurement Science
State-of-the-art laboratory facilities (>380!)
World leading National Measurement Institute
About NPL
1/17
Talk overview
Temperature measurement scale, and fixed-points
Limitations of thermocouples above 1100 °C
High-temperature fixed points
Concept of in-situ self-validation & implementation
Performance of self-validating thermocouples, up to 2300 ˚C
Overview
Further development & results
Industrial applications
Conclusions
2/17
Temperature measurement scale
Internationally agreed temperature
scale of 1990: “ITS-90”
Defined by a range of fixed-points
from…
The lowest vapour pressure point
of helium (-270.15 ˚C)
to…
The freezing point of copper
(1084.62 ˚C)
Further information: www.bipm.org/en/publications/its-90.html 3/17
Temperatures above 500 °C
There are only four temperature fixed points
defined by the ITS-90 above 500 ˚C:
• Freezing temp. Al 660.323 ˚C
• Freezing temp. Ag 961.78 ˚C
• Freezing temp. Au 1064.18 ˚C
• Freezing temp. Cu 1084.62 ˚C
Thermocouples provide the best uncertainties at
high temperatures (for contact thermometry)
How can we judge the performance of, and
calibrate contact sensors above 1100 ˚C ?
4/17
Limitations above 1100 °C
Lack of ITS-90 fixed points
W-Re thermocouples are commonly used
above 1500 ˚C
• Embrittlement
• Quickly exhibit thermoelectric drift –
typically 10 ˚C within 10 h of operation
• Recalibration often impossible
To address this issue, NPL are working in
cooperation with ESA-estec to develop an
innovative method of validating the
performance of high temperature
thermocouples in-situ
Further information: Brixy et al. High Temperatures – High Pressures 12, 625-631 (1994)
[After Brixy]
5/17
High-temperature fixed points
Novel high-temperature fixed points (HTFPs)
Many eutectic metal-carbon alloys have been
shown to be suitably stable as HTFPs, for
example:
• MP Fe-C 1153 ˚C
• MP Co-C 1324 ˚C *
• MP Pd-C 1492 ˚C
• MP Rh-C 1657 ˚C
• MP Pt-C 1738 ˚C *
• MP Ru-C 1953 ˚C *
• MP Ir-C 2292 ˚C *
• MP Re-C 2474 ˚C
• …
MP determined by radiation
thermometry
Reproducibility of the melting
point is known to be better than
±0.05 ˚C (k = 2)
6/17
Calibrations above 1100 °C
Co-C (1324 °C) & Pd-C (1492 °C)
HTFP construction:
Extremely pure metals
Graphite crucible – easy to manufacture
Tall for good immersion
Furnace:
High temperature furnace
Inert atmosphere (argon)
Measurement:
High temperature thermocouples
Materials compatibility issues
7/17
In-situ self-validation
Overcomes embrittlement
Is a technique to check the calibration of a
thermocouple in-situ
The design consists of two parts:
• Miniature HTFP (containing a eutectic
M-C ingot)
• High temperature thermocouple
(Type C, W5%Re-W26%Re)
By incorporating a miniature HTFP cell onto
the thermocouple tip, the thermovoltage can
be verified each time the thermal environment
passes the fixed point transition temperature
8/17
Implementation
Thermocouple construction crucial – we would like to thank Omega Engineering
for supplying thermocouples with a customised design
9/17
Position the HTFP onto the
thermocouple in-situ
Observe thermocouple output
through the transition temperature
Apply suitable correction algorithm
to the output reference function
Assured measurement
confidence and
extended useful life
Choose a HTFP – with a transition
temperature to match process The user is enabled to
perform a suitable
adjustment to the
reference function
(can be automated)
Prototype Test Arrangement
HTFP cell positioned on Type C thermocouple
Thermal cycling
• Ramp rate of 1 ˚C/min
• Held at maximum temperature for 1 hour
• Presence of cell does not impede sensor
function (under these conditions)
Melting temperature assigned by radiation
thermometry (ISO17025, traceable to ITS-90)
10/17
HTFP alloy Melting temperature,
°C
Uncertainty
(k = 2), °C
Co-C 1323.28 0.64
Pt-C 1737.52 0.94
Ru-C 1952.98 1.00
Ir-C 2289.70 1.56
11/17
Performance
The performance of the four self-validating thermocouples:
11/17
Performance
The performance of the four self-validating thermocouples:
At high temperatures, the thermocouple reading is clearly unreliable
11/17
Performance
The performance of the four self-validating thermocouples:
At high temperatures, the thermocouple reading is clearly unreliable
Ru-C drift between 1st and 2nd melt is: 20.9 μV (~1.7 ˚C)
Overview
By correcting for every step in drift, the user gains confidence in the
temperature reading
• The thermocouple is kept within calibration: extending its useful life
• The uncertainty due to changing thermoelectric homogeneity is eliminated
Design and use can be tailored for specific requirements
Development areas:
• One ingot (fixed-point temperature) and therefore limited temperature
range of correction validity
• Short term exposure
• Ingot size
• Thermocouple and HTFP cell are separate items (at present)
12/17
First multi-cell results
Multi-cells to allow dual validation; contains both Pt-C and Ru-C
Each ingot is clearly observed, therefore
Good thermal contact has been achieved 13/17
14/17
Extended exposure
Extending the exposure to 10 h at high temperature:
Maximum temperature maintained for 10 hours
Thermovoltage drift over 5 h (at Ir-C) is 347 µV, equivalent to ~43 ˚C
Industrial applications
Benefits to industry are clear
• Improved temperature measurement and reliability
• Casting / Manufacturing – reduced costs and enhanced quality
Looking to develop into a commercial device
• User requirements
• Develop correction algorithm
Nuclear industry – self-validation with low neutron
capture cross-section materials e.g. Fe-C and Cu
(EMRP project “MetroFission”)
15/17
Conclusions
The thermocouple and HTFP arrangement has been
shown to be suitable for self-validation
• Cell size provides suitable immersion
• The presence of the cell does not impede on
the function of the sensor, under these
conditions
Application of in-situ self-validation will achieve:
• Assured temperature measurement confidence
• Extended useful life of the sensor
Which opens up the possibility for:
• Improved temperature measurement /reliability
• Reduced costs and enhanced quality
16/17
HTFP = 1323.28 ˚C ± 0.64 ˚C HTFP = 1737.52 ˚C ± 0.94 ˚C
HTFP = 1952.98 ˚C ± 1.00 ˚C HTFP = 2289.70 ˚C ± 1.56 ˚C
