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 T90 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)/dT around TTPW. 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-T90) 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)/dT at TTPW 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),