Integrated self-validating thermocouples with a reference temperature up to 1329 °C

Abstract

Thermoelectric instability, or calibration drift, is a major problem for users of thermocouples at high temperature. NPL, in collaboration with CCPI Europe, AFRC and ICPE-CA/BRML-INM has designed, made and industrially tested Type S thermocouples with integrated temperature fixed-point cells. These in situ, self-validating thermocouples (denoted 'inseva') have the same external dimensions as conventional industrial thermocouples (the recrystallised alumina sheath has an outer diameter 7 of mm). The device can be used to detect the melting and/or freezing temperature of the integrated temperature fixed-point ingot, which enables a self-validation to be performed whilst in situ. During the testing, three different reference ingots were used in the cells, namely: copper (1084 °C), cobalt-carbon (1324 °C) and nickel-carbon (1329 °C). The metrological performance for two iterations of the design are presented, with an emphasis on the ability of the inseva thermocouples to indicate their own thermoelectric stability. A measurement uncertainty budget is also given for the case of a Ni-C inseva thermocouple. This paper demonstrates that inseva thermocouples can be successfully validated during industrial processes through the observation of the melting plateau, as well as their robustness over time in industrial conditions. A key finding is that the Ni-C eutectic alloy is much more robust than the Co-C eutectic alloy for the used type of graphite crucible, making the Ni-C inseva thermocouple more suitable for industrial applications, and a good alternative.

Digital twins for metrology; metrology for digital twins

“…The requirements for high precision measurement on the factory floor and for reliable measurement in challenging environments has led metrologists to develop approaches to in situ realisation of SI units [29][30][31] and self-validation of sensors [32,33]. This approach shrinks the traceability chain and removes the need for calibration (or allows for remote or self-calibration), but it is likely that some monitoring of these in situ realisations will be necessary to maintain end user confidence in their stability and accuracy.…”

Progress towards in-situ traceability and digitalization of temperature measurements

“…A typical output of a self-validating thermocouple during the recalibration process is shown in the lower panel of Figure 1. This device has been extensively characterised [6] A similar concept has been employed for an application in space-borne instrumentation, where the phase-change cell is part of the system whose temperature is to be measured. Such an embedded fixed-point has been demonstrated by NPL in collaboration with RAL Space on a prototype blackbody calibrator designed for operation as part of a spacecraft-borne earth observation instrument suite [5].…”

Mathematical Study of Temperature Measurement by Thermocouple in Chemical Processes

“…In this case, smaller values of temperature inclinations can be balanced by adding σ T . The smallest area in each part mentioned here varies according to the farthest distance (≈30,0mm) from the exchange center of the thermocouple 8 .…”