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

• DOI: 10.1007/s10765-016-2176-4
• Publication Date: 2017-01-13T05:28:48Z
• Authors: R. Underwood; M. de Podesta; G. Sutton; L. Stanger; R. Rusby; P. Harris; P. Morantz; G. Machin
• Keywords: Triple point; Atmospheric temperature range; Classification of discontinuities

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 $$T_\mathrm{TPW}$$ .