An improved methodology for the measurement of insulation resistance of MIMS thermocouples at high temperatures: The time-dependent behavior of the insulation resistance

The current methodology suggested by technical standards for the assessment of the insulation resistance of thermocouple cables at high temperatures relies on the use of a high voltage source between the wires or between a wire and the sheath of the sensor. Such a methodology is suggested by technical standards to test the insulation resistance at both ambient temperature and high temperatures. In this paper the authors argue that an alternative methodology based on the use of a current source is preferable for the measurements of insulation resistance at high temperatures. The new methodology was applied in this work to measure the insulation resistance of type K thermocouples between 800 °C and 1300 °C. Long-term tests in both isothermal and thermal cyclic conditions were undertaken: these tests showed a time- dependent change of the insulation resistance at high temperatures, a phenomenon