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Relating composition and thermelectric stability of Pt-Rh alloy thermocouples.

Pearce, J V; Greenen, A D; Smith, A; Elliott, C J (2017) Relating composition and thermelectric stability of Pt-Rh alloy thermocouples. Int. J. Thermophysics, 38 (2). p. 26.

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A simple model is presented which relates the electromotive force (emf) drift rate of Pt-Rh thermoelements to dS/dc, the sensitivity of the Seebeck coefficient, S, to rhodium mass fraction, c. The model has been tested by repeated measurements of a Pt-Rh thermocouple assembly consisting of five thermoelements, using a Co-C high temperature fixed point (1324 °C) for a total duration of 500 hours. By considering various thermocouples from the assembly, it is demonstrated that in this case, remarkably, there is a linear relationship between the measured drift rate and the combined dS/dc, where the combination is determined by addition of the individual values for each wire. Particular emphasis is placed on evaluation of the uncertainties associated with the calculations. This result supports previous findings that the thermoelectric stability of Pt-Rh thermoelements improves as the rhodium mass fraction increases. Within this paradigm, it is shown that for a selected Pt-Rh thermoelement of any given composition, there exists a second thermoelement having a composition that yields a thermocouple with zero drift. The results are applicable only to use at 1324 °C; further work to explore the wider applicability of the conclusions is planned within the European Metrology Programme for Innovation and Research project 14IND04 (EMPRESS).

Item Type: Article
Keywords: Thermocouple, HTFP, stability, temperature, Pt-Rh
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Divisions: Engineering, Materials & Electrical Science
Identification number/DOI: 10.1007/s10765-016-2159-5
Last Modified: 02 Feb 2018 13:13
URI: http://eprintspublications.npl.co.uk/id/eprint/7431

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