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Relating Composition and Thermoelectric Stability of Pt–Rh Alloy Thermocouples

  • J. V. Pearce
  • A. D. Greenen
  • A. Smith
  • C. J. Elliott
TEMPMEKO 2016
Part of the following topical collections:
  1. TEMPMEKO 2016: Selected Papers of the 12th International Symposium on Temperature, Humidity, Moisture and Thermal Measurements in Industry and Science

Abstract

A simple model is presented which relates the electromotive force 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 (\(1324{\,}^{\circ }{\mathrm{C}}\)) for a total duration of 500 h. 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 minimum drift when combined with the first to form a thermocouple.

Keywords

HTFP Pt–Rh Stability Thermocouple Thermoelectric stability 

Notes

Acknowledgements

AS was a guest worker at NPL for 2 months in 2012 under the auspices of the UK Southeast Physics network (SEPnet) scheme. JP acknowledges funding from the UK National Measurement System and helpful discussions with Richard Rusby. Karen Alston is thanked for assistance with the multi-wire thermocouple preparation. Part of this work was carried out as part of a European Metrology Program for Innovation and Research (EMPIR) project to enhance process efficiency through improved temperature control, ‘EMPRESS’. The EMPIR is jointly funded by the EMPIR participating countries within EURAMET and the European Union. ©Crown Copyright 2016. Reproduced by permission of the Controller of HMSO and the Queen’s printer for Scotland.

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Copyright information

© Crown Copyright 2016

Authors and Affiliations

  1. 1.National Physical LaboratoryTeddingtonUnited Kingdom

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