Thermoelectric stability of dual-wall and conventional type K and N thermocouples

Mineral insulated metal sheathed (MIMS) base metal thermocouples experience thermoelectric drift over their lifetime caused by use at high temperatures and metallurgical changes, causing spurious measurement errors. CCPI Europe Limited and University of Cambridge have designed a MIMS thermocouple with an additional inner sheath, in order to protect the thermoelements from the effects that cause thermoelectric drift. The performance of these dual-wall thermocouples and conventional type N and type K thermocouples are assessed at six different National Metrology Institutes (NMIs) using two different testing regimes: isothermal testing at 1200 degrees C, and thermal cycling tests between 300 degrees C and 1150 degrees C. The investigation demonstrates that in both testing regimes, the type N dual-wall thermocouples showed a significantly reduced thermoelectric drift by about a factor of three compared to the conventional thermocouples. There was no significant difference between the type K dual-wall and conventional type K thermocouples in the isothermal tests, and the type K dual-wall thermocouples showed greater drift than the conventional thermocouples in the thermal cycling tests, but the conventional type K thermocouples were less robust than the dual-wall type K thermocouples. The results presented in this paper represent an impartial assessment of the thermoelectric stability of both dual-wall thermocouples and conventional thermocouples, which may provide assurance to potential users.