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Stability assessment of a new high strength, high temperature Type R and Type S thermocouple

Pearce, J V; Tucker, D; Rawal, R; Hutton, L (2020) Stability assessment of a new high strength, high temperature Type R and Type S thermocouple. Measurement Science and Technology, 31 (4). 044005 ISSN 0957-0233

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Abstract

Platinum-rhodium Type R and S thermocouples fail in service because of two main reasons: excessive grain growth in the platinum limb at high temperatures (causing wire breakage) and temperature drift, i.e. deviation of electromotive force (emf) generated over time because of deposition of rhodium oxide on the platinum limb, out of accepted industry tolerances. Finding a solution to the problem is challenging because of the contradictory requirements of strength and narrow permissible emf range: emf is extremely sensitive to any dopants or alloying elements that can be used to improve strength at high temperatures. Johnson Matthey has developed a new thermocouple wire (platinum limb) called `HTX™' by doping platinum with a small quantity of zirconium which is oxidised during processing to electrically neutral zirconia. Zirconia improves the high temperature strength of platinum wire by restricting the grain growth but does not significantly affect the emf. The result is a wire which has the potential to last many times longer at high temperatures (>1200 °C) than standard platinum wire, whilst still achieving Class 1 tolerance (i.e. ± 1 °C at 1000 °C). It also improves the thermoelectric drift characteristics of the thermocouple by counteracting any reduction in emf due to rhodium contamination by an increase in emf as any remaining zirconium is converted to zirconia. We describe a long-term assessment (up to 1500 hours) of the thermoelectric stability of the HTX¿ wire by exposure to high temperature with periodic in-situ calibration using a Co-C (1324 °C) high temperature fixed point. This yields stability measurements with very high precision. In addition, we describe high resolution measurements of the thermoelectric homogeneity as the test progresses, to provide vital information on drift mechanisms. This information, combined with microscopy, is employed to assess the metallurgical and transport behaviour of zirconium in the HTX wire.

Item Type: Article
Keywords: Thermocouple, noble metal, platinum, ITS-90, thermometry
Subjects: Engineering Measurements > Thermal
Divisions: Engineering
Identification number/DOI: 10.1088/1361-6501/ab48bd
Last Modified: 29 May 2020 13:55
URI: http://eprintspublications.npl.co.uk/id/eprint/8710

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