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Thermal recovery from cold-working in type K bare-wire thermocouples

Greenen, A; Webster, E S (2017) Thermal recovery from cold-working in type K bare-wire thermocouples. International Journal of Thermophysics, 38 (12). 179

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Cold-working of most thermocouples has a significant, direct impact on the Seebeck coefficient which can lead to regions of thermoelectric inhomogeneity and accelerated drift. Cold-working can occur during the wire swaging process, when winding the wire onto a bobbin or during handling by the end user - either accidentally or deliberately. Swaging-induced cold-work in thermocouples, if uniformly applied, may result in a high level of homogeneity. However, on exposure to elevated temperatures, the subsequent recovery process from the cold-working can then result in significant drift; and this can in turn lead to erroneous temperature measurements, often in excess of the specified manufacturer tolerances. Several studies have investigated the effects of cold-work in type K thermocouples usually by bending, or swaging. However, the amount of cold-work applied to the thermocouple is often difficult to quantify, as the mechanisms for applying the strains are typically non-linear when applied in this fashion.

A repeatable level of cold-working is applied to the different wires using a tensional loading apparatus to apply a known yield displacement to the thermoelements. The effects of thermal recovery from cold-working can then be accurately quantified as a function of temperature, using a linear gradient furnace and a high-resolution homogeneity scanner. Variation of these effects due to differing alloy compositions in type K wire is also explored, which is obtained by sourcing wire from a selection of manufacturers. The information gathered in this way will inform users of type K thermocouples about the potential consequences of varying levels of cold-working, and its impact on the Seebeck coefficient at a range of temperatures between ~70 °C and 600 °C. This study will also guide users on the temperatures required to rapidly alleviate the effects of cold-working using thermal annealing treatments.

Item Type: Article
Keywords: Thermocouple Cold-Work Inhomogeneity Type K Base Metal
Subjects: Engineering Measurements > Thermal
Divisions: Engineering, Materials & Electrical Science
Identification number/DOI: 10.1007/s10765-017-2316-5
Last Modified: 03 May 2018 14:07
URI: http://eprintspublications.npl.co.uk/id/eprint/7914

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