By using a simple model to relate the electromotive force drift rate of Pt–Rh thermoelements to dS/dc, i.e. the sensitivity of the Seebeck coefficient, S, to rhodium mass fraction, c, the composition of the optimal pair of Pt–Rh wires that minimizes thermoelectric drift can be determined. The model has been applied to four multi-wire thermocouples each comprising 5 or 7 Pt–Rh wires of different composition. Two thermocouples were exposed to a temperature of around 1324 °C, one thermocouple to around 1492 °C, i.e. the melting points of the Co–C and Pd–C high temperature fixed points, respectively, and one thermocouple to a series of temperatures between 1315 °C and 1450 °C. The duration of exposure at each temperature was several thousand hours. By performing repeated calibrations in situ with the appropriate fixed point during the high temperature exposure, the drift performance has been quantified with high accuracy, entirely free from errors associated with thermoelectric homogeneity. By combining these results it is concluded that the Pt-40%Rh versus Pt-6%Rh is the most stable at the temperatures investigated. A preliminary reference function was determined and is presented.

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Thermocouples are widely used as temperature sensors in industry. The electromotive force generated by a thermocouple is produced in a temperature gradient and not at the thermocouple tip. This means that the thermoelectric inhomogeneity represents one of the most important contributions to the overall measurement uncertainty associated with thermocouples. To characterise this effect, and to provide some general recommendations concerning the magnitude of this contribution to use when formulating uncertainty analyses, a comprehensive literature survey has been performed. Significant information was found for Types K, N, R, S, B, Pt/Pd, Au/Pt and various other Pt/Rh thermocouples. In the case of Type K and N thermocouples, the survey has been augmented by a substantial amount of data based on calibrations of mineral-insulated, metal-sheathed thermocouple cable reels from thermocouple manufacturers. Some general conclusions are drawn and outline recommendations given concerning typical values for the uncertainty arising from thermoelectric inhomogeneity for the most widely used thermocouple types in the as-new state. It is stressed that these recommendations should only be heeded when individual homogeneity measurements are not possible. It is also stressed that the homogeneity can deteriorate rapidly during use, particularly for base metal thermocouples.

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.

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