National Physical Laboratory

Self-calibrating thermocouples for accurate high temperature measurement

Summary

Self-calibrating thermocouple

NPL's temperature scientists have developed a self-validating thermocouple technology, which incorporates 'fixed-points' (materials which melt at well defined temperatures) made of metal-carbon eutectic alloys. This means that the thermocouple is able to self-validate, or check, that it is measuring the temperature correctly, by adjusting its calibration in response to the melting of the fixed point alloy. This is great news for industrial thermocouple users that rely on these thermocouples for process control.

Background

Current high temperature (above 1100 °C) thermocouples become less and less reliable as the temperature increases – in extreme cases, at very high temperatures (>1750 °C) they can be measuring the temperature incorrectly by as much as tens of degrees. When these thermocouples are used in precision manufacturing facilities, such as heat-treatment furnaces for high performance turbine components, even a 5 °C error is unacceptable and could lead to substantial waste, because the components produced would not be suitable for use.

What is needed, then, is a thermocouple which 'checks itself' as this would confirm whether it is measuring the correct temperature or not - giving users increased confidence in its reading. In NPL's new thermocouple technology, temperature fixed points comprising metal-carbon eutectic alloys form an integral part of the thermocouple, either as part of the measuring junction, or as a conventional micro-crucible surrounding the measuring junction.

The melting and freezing plateaux of the eutectic alloy are observable as a 'hesitation' in the thermocouple output as the temperature of the measuring junction passes through the fixed-point temperature. As this fixed-point temperature is well defined, the thermocouple can self-adjust its calibration, thereby mitigating any drift which may have taken place.

NPL applies this technique of self-validation to Pt/Pt-Rh thermocouples (Types R, S, and B), Pt/Pd thermocouples, and W-Re thermocouples (Type C). The thermocouples developed so far include the metal-carbon eutectics Co-C and Pt-C. Nonetheless most thermocouple types, and most metal-carbon eutectic alloys and metal (carbide)-carbon eutectic alloys can be used as the fixed point.

Impact

Many high technology industrial sectors, including aerospace, materials processing, and nuclear fuel production, operate at temperatures in excess of 1500 °C and would benefit from these improvements.

In gas turbines, the need has been highlighted for improved temperature measurement in the combustion system (up to 2400 °C) and high pressure turbines (up to 1800 °C), to improve fuel efficiency and reduce emissions while continuing to operate safely.

In materials processing, the ceramics industry require improved temperature measurement up to 2200 °C, with sintering of, for example, SiC and graphitisation of carbon fibres up to 2700 °C to 3000 °C.

In the nuclear industry, the quality of nuclear fuel is critically dependent upon the sintering temperature of the fuel elements at ~1750 °C. This is currently controlled by tungsten-rhenium thermocouples which are prone to drift because of the hostile environment and the high temperatures to which they are exposed.

These processes all provide examples of how better temperature measurement, facilitated by self-validation, will make a significant improvement in industrial measurement, for production of a more consistent product, reduction of fuel consumption and CO2 emissions, and increased safety.

For further information, please contact Jonathan Pearce

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Last Updated: 10 Oct 2012
Created: 12 Nov 2010

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