Further Estimates of (T-T90) Close to the Triple Point of Water

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 TPW = 273.16 K. In that work, the T 90 measurements below T TPW used a different ensemble of capsule standard platinum resistance thermometers (SPRTs) than the T 90 measurements above T TPW . In this work, we extend our earlier measurements using the same ensemble of SPRTs above and below T TPW , enabling a deeper analysis of the slope d ( T - T 90 ) / d T around T 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 d ( T - T 90 ) / d T at T 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 d ( T - T 90 ) / d T at T TPW .