Low-Temperature Drift in MIMS Base-Metal Thermocouples

Inhomogeneities are known to develop within thermoelements exposed to elevated temperatures, resulting in temperature measurement errors. While the Seebeck coefficient drift in base-metal thermocouples due to aging at temperatures over \(200\,^{\circ }\mathrm{C}\) has been extensively investigated, there have been very few investigations into possible Seebeck changes at lower temperatures. Despite warnings about possible effects, most practitioners assume changes in homogeneity are either not significant or not able to develop at temperatures less than \(200\,^{\circ }\mathrm{C}\) . This study reports on measurements of inhomogeneities in base-metal thermocouples arising from heat treatment at temperatures in the region of \(200\,^{\circ }\mathrm{C}\) . Thermoelectric scans of thermocouples were carried out following exposure of a range of mineral-insulated metal-sheathed base-metal thermocouples, from two large manufacturers, of Types E, J, K, N, and T, to either a linear-gradient furnace within the range of \(100\,^{\circ }\mathrm{C}\) to \(320\,^{\circ }\mathrm{C}\) or uniform temperature zones of \(100\,^{\circ }\mathrm{C}\) , \(150\,^{\circ }\mathrm{C}\) , and \(200\,^{\circ }\mathrm{C}\) . The experiments reveal noticeable drift in all base-metal types for temperatures as low as \(100\,^{\circ }\mathrm{C}\) and exposure times as short as 1 h. The most sensitive thermoelement alloys appear to be Constantan, Alumel, and Nicrosil. It is concluded that the common working assumption that base-metal thermocouples suffer no thermally induced changes in the Seebeck coefficient below \(200\,^{\circ }\mathrm{C}\) is false. This observation has significant implications for many high-stability monitoring and control systems reliant on base-metal thermocouples that operate in the range of \(100\,^{\circ }\mathrm{C}\) to \(200\,^{\circ }\mathrm{C}\) . Additionally, thermoelectric scanning of base-metal thermocouples should be carried out at temperatures well below \(150\,^{\circ }\mathrm{C}\) to avoid erasure of strain effects or imprinting of new thermal signatures.