Pearce, J; Tucker, D; Izquierdo, C G; Caballero, R; Ford, T; Williams, P; Cowley, P (2022) Correlation between insulation resistance and temperature measurement error in Type K and Type N mineral insulated, metal sheathed thermocouples. International Journal of Thermophysics, 43. 35

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Abstract

Mineral insulated, metal sheathed (MI) Type K and Type N thermocouples are widely used in industry for process monitoring and control. One factor that limits their accuracy is the dramatic decrease in the insulation resistance at temperatures above about 600 °C which results in temperature measurement errors due to electrical shunting. In this work the insulation resistance of a cohort of representative MI thermocouples was characterised at temperatures up to 1160 °C, with simultaneous measurements of the error in indicated temperature by in situ comparison with a reference Type R thermocouple. Intriguingly, there appears to be a systematic relationship between the insulation resistance and the error in the indicated temperature. At a given temperature, as the insulation resistance decreases, there is a corresponding increasingly negative error in the temperature measurement. Although the measurements have a relatively large uncertainty, the trend is apparent at all temperatures above 600 °C, which suggests that it is real. Furthermore, the correlation disappears at temperatures below about 600 °C, which is consistent with the well-established diminution of insulation resistance breakdown effects below that temperature. This raises the intriguing possibility of using the as-new MI thermocouple calibration as an indicator of insulation resistance breakdown: large deviations of the electromotive force (emf) in the negative direction could indicate a correspondingly low insulation resistance.

Item Type:	Article
Keywords:	Insulation resistance breakdown, Mineral insulated metal sheathed thermocouple, Temperature, Thermocouple, Thermoelectric
Subjects:	Engineering Measurements > Thermal
Divisions:	Thermal & Radiometric Metrology
Identification number/DOI:	10.1007/s10765-021-02967-x
Last Modified:	02 Feb 2022 15:23
URI:	https://eprintspublications.npl.co.uk/id/eprint/9335