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Thermal Preconditioning of MIMS Type K Thermocouples to Reduce Drift

  • E. S. WebsterEmail author
Article

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.

Keywords

Annealing Drift Homogeneity scanning Inhomogeneity Preconditioning Thermocouples Type K 

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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Measurement Standards LaboratoryLower HuttNew Zealand

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