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.

开尔文 (K) 是热力学温度的 SI 基本单位。在 2018 年 11 月举行的第 26 次会议上，度量衡大会 (CGPM) 批准重新定义开尔文，将玻尔兹曼常数 ( k )的值固定为 1.380649 × 10 ^-23，以单位 JK ^{-1 表示}，这是在水的三相点温度 (TPW, 273.16 K) 下测量的，以保持与旧定义的一致性。重新定义的 SI 单位从 2019 年 5 月 20 日起在全球范围内实施。 尽管如此，TPW 仍然是非常重要和基本的固定点，对于热力学 ( T ) 和 1990 年实用国际温标 (ITS-90, T ₉₀)，其中热力学和实用尺度具有相等的价值。在 CSIR-NPL，我们正在开发声学气体温度计 (AGT)，以实现基于玻尔兹曼常数的新开尔文并传播热力学温度。对于相对 AGT 和在各种 ITS-90 子范围内校准胶囊 SPRT，TPW 的测量能力需要具有非常低的不确定性。为此，我们使用长杆 SPRT 和高精度直流电阻测温桥 (0.02 ppm) 实现了一个由三个 TPW 单元组成的集合，这些单元具有已知的同位素组成和与维也纳标准平均海水 (VSMOW) 的偏差，具有温度-稳定的电阻标准。TPW 电池中的冰幔是通过多次插入在液氮中冷却的金属棒 (LN ₂)，并在测量前退火约 48 小时。所有相关的不确定性参数，例如重复性、再现性、同位素和杂质、残余气体压力、静水压头、热通量浸入、SPRT 自加热、光管道、电阻和电桥测量，都经过仔细评估，并计算了扩展不确定性 对于细胞 C1、C2 和 C3，分别为 ± 57 µK、± 60 µK 和 ± 58 µK（k = 2）。获得的 TPW 实现不确定性与既定的 TPW 国际比较相称。这组 TPW 单元被指定为国家参考，并成功地在印度国家计量研究所 (NMI) 的 CSIR-NPL 提供温度传播服务。