Correlation Between Insulation Resistance and Temperature Measurement Error in Type K and Type N Mineral Insulated, Metal Sheathed Thermocouples

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 (up to about 1 °C in temperature error and
up to about 10 % in insulation resistance measurement), 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.