Thermal Preconditioning of MIMS Type K Thermocouples to Reduce Drift

Abstract

Type K thermocouples are the most widely used temperature sensors in industry and are often used in the convenient mineral-insulated metal-sheathed (MIMS) format. The MIMS format provides almost total immunity to oxide-related drift in the 800 ${^{\circ }}\hbox {C}$–1000 ${^{\circ }}\hbox {C}$ range. However, crystalline ordering of the atomic structure causes drift in the range 200 ${^{\circ }}\hbox {C}$–600 ${^{\circ }}\hbox {C}$. Troublesomely, the effects of this ordering are reversible, leading to hysteresis in some applications. Typically, MIMS cable is subjected to a post-manufacturing high-temperature recrystallization anneal to remove cold-work and place the thermocouple in a ‘known state.’ However, variations in the temperatures and times of these exposures can lead to variations in the ‘as-received state.’ This study gives guidelines on the best thermal preconditioning of 3 mm MIMS Type K thermocouples in order to minimize drift and achieve the most reproducible temperature measurements. Experimental results demonstrate the consequences of using Type K MIMS thermocouples in different states, including the as-received state, after a high-temperature recrystallization anneal and after preconditioning anneals at 200 ${^{\circ }}\hbox {C}$, 300 ${^{\circ }}\hbox {C}$, 400 ${^{\circ }}\hbox {C}$ and 500 ${^{\circ }}\hbox {C}$. It is also shown that meaningful calibration is possible with the use of regular preconditioning anneals.