Recent developments in high temperature sensing
Jonathan V. Pearce
Senior research scientist, National Physical Laboratory (NPL), UK
Graham Machin
Head, Temperature Standards, National Physical Laboratory (NPL), UK
Synopsis: Some dramatic improvements in thermometry above 1100 °C, which will be of considerable interest to industrial
users, are discussed. These include a new generation of high temperature fixed points (for calibration of thermocouples) that are
based on melting of metal-carbon eutectic alloys, a new ultra-stable thermocouple based on the pure metal thermoelements Pt
and Pd, and recent developments in self-validating thermocouple technology and radiation thermometry.
Currently, industrial temperature measurements are mainly based on the traditional
Pt-Rh (Types R, S, and B) thermocouple standards, calibrated at the highest
temperature using the palladium wire point (1554 °C). This is achieved by welding a
1 cm piece of Pd wire across the two legs of the thermocouple; the emf at which this
wire melts provides the calibration. Pt-Rh sensors are employed in the majority of
power and process applications, such as power generation plants, aerospace heat
treatment plants, and gas turbines up to about 1600 °C.
However, a step change improvement in high temperature measurement is
taking place across the world, with NPL in the vanguard. This is largely due to a new
generation of high temperature fixed points (HTFPs), which are small ingots with
invariant melting temperatures above the copper fixed point (1084 °C).
At their simplest HTFPs are binary eutectic alloys of metal and carbon,
examples of which are given in the table below. Research into HTFPs for calibration
of both non-contact (radiation) and contact (thermocouple) thermometers has
advanced rapidly since their inception by Yoshiro Yamada of the National Metrology
Institute of Japan (NMIJ), and their performance has now been well characterised.
The repeatability, i.e. the measured temperature of repeated melts, for all the HTFPs
is better than 0.05 °C. The reproducibility (agreement between cells from different
suppliers) is also now so good that demonstrating reproducibility is approaching the
limits of current measurement.
Eutectic
Fe-C
Co-C
Ni-C
Pd-C
Rh-C
Pt-C
Ru-C
Ir-C
Re-C
Temperature /
°C
1153
1324
1329
1492
1657
1738
1953
2292
2474
HTFPs with nominal melting temperatures.
HTFPs for thermocouple calibration
A strong body of research is underway around the world to establish first Co-C and
then Pd-C as fixed points for thermocouple calibrations. In the next few years, when
internationally agreed temperatures are assigned to the HTFPs, ingots for
thermocouple calibration will supplant the wire-bridge method of calibration. There
are three reasons for this:
a)
b)
c)
the wire bridge method involves destruction of the measuring junction of
the thermocouple, not necessary for an ingot based technique
the Co-C point is mid-way between the Cu point and the Pd wire bridge
point and so is ideally located to minimise interpolation uncertainty
the uncertainty around the highest temperature will be routinely 0.5 °C,
compared with 1.5 °C as at present.
Within the UK, NPL already has UKAS accredited calibration capability at the Co-C
point, and it has also been installed in an industrial setting on a trial basis for over two
years. Furthermore, a formal comparison of independently constructed Co-C fixed
points between the national measurement institutes in the UK, France, Germany and
Japan has yielded a maximum difference in the melting temperature of < 0.1 °C.
The technical challenges associated with the robustness and Pd vapour
transport of the Pd-C fixed point have now been overcome, and the Pd-C point now
comprises a second UKAS accredited thermocouple calibration point.
Higher temperature HTFPs can be used for the calibration of refractory metal
thermocouples such as W/Re (Types C and D), and trials have already been
performed in this area. The temperature values for all the fixed points are determined
using radiation thermometry.
1.6
Type R (fixed points)
Type R (wire bridge)
Uncertainty (k = 2) / °C
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
1000
1200
1400
Temperature / °C
Calibration uncertainty of Type R thermocouples, using eutectic fixed point-based
calibration and wire bridge calibration.
Pt/Pd thermocouples
The pure thermoelement Pt/Pd thermocouple is coming of age, offering substantially
improved stability, repeatability and homogeneity up to 1500 °C, and it has been
standardised in BS EN 62460:2008. The advent of HTFPs has made the accurate
calibration of Pt/Pd thermocouples possible, giving access to its superior performance.
Traditionally, Pt/Pd thermocouples have been alumina sheathed sensors with
relatively thick (e.g. 0.5 mm) pure metal thermoelements joined at the hot junction by
a stress relieving coil of very fine platinum (typically 0.1 mm to 0.2 mm). This coil is
to allow for the mismatch in thermal expansivities of the pure metal thermoelements,
thus preventing strain from causing a failure of the measuring junction. While the coil
increases the sensor lifetime, it also makes the thermometer very fragile, limiting its
use to specialist laboratory applications.
However, recent design innovations by NPL, without the stress-relieving coil,
have been trialled that demonstrate similar performance of the Pt/Pd thermocouple to
the original design. These developments open up interesting new application
possibilities. For example, a rugged version has been developed in conjunction with
industrial partners and this is currently undergoing long-term drift and robustness tests
in the furnace of a commercial heat treatment provider. Calibration of these sensors at
NPL is now accredited by UKAS.
As well as being a top-level device for transferring temperature scales between
national metrology institutes (NMIs), in the longer term it is envisaged that use of
Pt/Pd thermocouples in industry will become widespread, with considerable benefits
for process control.
Type R (fixed points)
Pt/Pd (fixed points)
Uncertainty (k = 2) / °C
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
200
400
600
800
1000
1200
1400
Temperature / °C
22684
Melt
Freeze
0.1 °C
22682
Emf / µV
22680
22678
22676
22674
22672
0
100
200
300
400
500
600
Time / hours
The two key advantages of Pt/Pd thermocouples – stability and low uncertainty. Top:
comparison of the uncertainty of Pt/Pd calibration, using Zn, Ag, and Pd-C fixed
points, compared with Type R. Bottom: stability of Pt/Pd thermocouple as measured
by repeated melts and freezes at the Pd-C fixed point.
Self-calibrating high temperature thermocouples
The reliable use of noble metal thermocouples is limited to approximately 1600 °C.
For measurement of higher temperatures, the only solution currently, if radiation
thermometry is not feasible, is to use W/Re type thermocouples, but these are prone to
substantial drift and damaging embrittlement. A possible new solution is through
‘self-validation’. This has been implemented at lower temperatures (< 1000 °C) for
thermocouples and platinum resistance thermometers, for optimising temperature
control in power generation, but not, until now, for W/Re sensors. Self-validation can
take the form of active control, where one or more fixed points are incorporated into
the measuring junction of the thermocouple. Every time the melting temperature is
passed, the thermocouple can be ‘recalibrated’ as the sensor output ‘hesitates’ while
the fixed-point material melts or freezes. The need for such procedures will become
increasingly urgent as energy costs rise and manufacturers are pressed to reduce
carbon footprints and shift to ‘zero-waste’ manufacture.
NPL has already developed prototype self-validating thermocouples
employing HTFPs, utilising miniature fixed point crucibles up to 1500 °C. Mineral
insulated, metal-sheathed (MIMS) type W/Re thermocouples have also been used
with HTFPs. This type of self-calibrating thermocouple is robust, and initial results
look very promising. Both designs permit the use of multiple fixed points, so a
crucible can contain any combination of materials, e.g. Ag (962 °C) and Pt-C (1738
°C), permitting self-calibration at two or more different temperatures.
17600
5 °C
17500
Emf / µV
962 °C
17400
17300
900
910
920
930
940
950
Time / minutes
Self-calibrating thermocouple. Top: W-Re thermocouple inserted in fixed point.
Bottom: Typical melting and freezing plateaux during cycling of the self-validating
W-Re thermocouple with silver fixed point, around the melting temperature.
Radiation thermometry
Many high temperature processes are measured and controlled by radiation
thermometry. The advent of HTFPs will facilitate improved traceability and process
control.
The temperature scale at high temperatures is currently disseminated to
industrial calibration laboratories through calibrated reference radiation thermometers.
Careful checks and precautions need to be implemented to identify if any drift has
occurred. This is essential to ensure the scale is realised in a reliable and robust way.
This can be avoided if the scale is instead transferred to the industrial laboratory
through calibrated HTFPs which are essentially driftless. Besides this obvious
advantage, very low uncertainties would, in principle, become achievable on a routine
basis by the calibration laboratory.
High temperature process control is often performed by radiation thermometry
through a window. Unfortunately, these often suffer from progressive contamination
necessitating routine cleaning and/or replacement. This problem could be overcome if
a HTFP was incorporated within the process. The radiation thermometer could at
intervals view the HTFP, and by monitoring its melt or freeze in-situ, the change in
window transmission can be routinely corrected for. This would ensure that processes
remained operating in optimal conditions for longer and with less intervention.
To summarise, it is clear that there are converging developments in the field of high
temperature metrology that are bringing about a step change improvement in high
temperature measurements. Adoption by industry will enable optimal energy use,
tighter process control, and reduced wastage.