Further Estimates of \((T-T_{90})\) Close to the Triple Point of Water

  • R. UnderwoodEmail author
  • M. de Podesta
  • G. Sutton
  • L. Stanger
  • R. Rusby
  • P. Harris
  • P. Morantz
  • G. Machin
Part of the following topical collections:
  1. TEMPMEKO 2016: Selected Papers of the 13th International Symposium on Temperature, Humidity, Moisture and Thermal Measurements in Industry and Science


Recent advances in primary acoustic gas thermometry (AGT) have revealed significant differences between temperature measurements using the International Temperature Scale of 1990, \(T_{90}\), and thermodynamic temperature, T. In 2015, we published estimates of the differences \((T-T_{90})\) from 118 K to 303 K, which showed interesting behavior in the region around the triple point of water, \(T_\mathrm{TPW}=273.16\) K. In that work, the \(T_{90}\) measurements below \(T_\mathrm{TPW}\) used a different ensemble of capsule standard platinum resistance thermometers (SPRTs) than the \(T_{90}\) measurements above \(T_\mathrm{TPW}\). In this work, we extend our earlier measurements using the same ensemble of SPRTs above and below \(T_\mathrm{TPW}\), enabling a deeper analysis of the slope \(\mathrm{d}(T-T_{90})/\mathrm{d}T\) around \(T_\mathrm{TPW}\). In this article, we present the results of seven AGT isotherms in the temperature range 258 K to 323 K. The derived values of \((T-T_{90})\) have exceptionally low uncertainties and are in good agreement with our previous data and other AGT results. We present the values \((T-T_{90})\) alongside our previous estimates, with the resistance ratios W(T) from two SPRTs which have been used across the full range 118 K to 323 K. Additionally, our measurements show discontinuities in \(\mathrm{d}(T-T_{90})/\mathrm{d}T\) at \(T_\mathrm{TPW}\) which are consistent with the slope discontinuity in the SPRT deviation functions. Since this discontinuity is by definition non-unique, and can take a range of values including zero, we suggest that mathematical representations of \((T-T_{90})\), such as those in the mise en pratique for the kelvin (Fellmuth et al. in Philos Trans R Soc A 374:20150037, 2016. doi: 10.1098/rsta.2015.0037), should have continuity of \(\mathrm{d}(T-T_{90})/\mathrm{d}T\) at \(T_\mathrm{TPW}\).


Acoustic gas thermometry ITS-90 Thermodynamic temperature 



This work was partly funded by the EMRP. The EMRP is jointly funded by the EMRP 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

© Springer Science+Business Media New York 2017

Authors and Affiliations

  • R. Underwood
    • 1
    Email author
  • M. de Podesta
    • 1
  • G. Sutton
    • 1
  • L. Stanger
    • 1
  • R. Rusby
    • 1
  • P. Harris
    • 1
  • P. Morantz
    • 1
  • G. Machin
    • 1
  1. 1.National Physical LaboratoryTeddingtonUK

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