Improved Realization of Ensemble of Triple Point of Water Cells at CSIR-NPL

Kelvin (K) is SI base unit of thermodynamic temperature. In its 26th meeting held on Nov 2018, General Conference on Weights and Measures (CGPM) has approved the redefinition of kelvin by fixing the value of the Boltzmann Constant (k) to be 1.380649 × 10−23 when expressed in the unit J K−1, which was measured at temperature of triple point of water (TPW, 273.16 K), to maintain the consistency with old definition. The redefined SI units are implemented worldwide from May 20, 2019. Still, TPW is very important and fundamental fixed point, for both, thermodynamic (T) and practical International Temperature Scale of 1990 (ITS-90, T90), where the thermodynamic and practical scale have the equal value. At CSIR-NPL, we are developing acoustic gas thermometry (AGT) for the realization of Boltzmann constant-based new kelvin and to disseminate the thermodynamic temperatures. For the relative AGT and for the calibration of capsule SPRTs at various ITS-90 sub-ranges, the measurement capability at TPW need to be of very low uncertainty. For this purpose, we have realized an ensemble of three TPW cells, of known isotopic composition and deviation with Vienna Standard Mean Ocean Water (VSMOW), using long-stem SPRTs and high-precision DC Resistance Thermometry Bridge (0.02 ppm) with temperature-stabilized resistance standards. Ice mantles in TPW cell were prepared using the multiple insertions of metal rods cooled in liquid nitrogen (LN2), and annealed for about 48 h before measurements. All the associated uncertainty parameters, such as repeatability, reproducibility, isotopic, and impurity, residual gas pressure, hydrostatic head, heat flux immersion, self-heating of SPRT, light piping, resistances and bridge measurements were carefully evaluated and expanded uncertainty was computed to be ± 57 µK, ± 60 µK and ± 58 µK, (k = 2) for cells C1, C2, and C3, respectively. The obtained TPW realization uncertainties are commensurate with the established international comparisons for TPW. This ensemble of TPW cells is designated as National Reference and successfully placed in temperature dissemination services at CSIR-NPL, National Metrology Institute (NMI) of India.