NRC measurement set-up and preparatory work for CCT-K7.2021 key comparison of triple-point-of-water cells

Abstract

This paper describes the NRC measurement set-up and the preparatory work for the CCT-K7.2021 key comparison of triple-point-of-water (TPW) cells. The preparatory work at NRC (charged with piloting the key comparison) included a comprehensive testing of the measurement set-up, the definition of the new NRC national standard for the TPW temperature, and the evaluation of the uncertainty budgets relevant for this key comparison. The new NRC TPW national standard is composed of an ensemble of 10 fused silica TPW cells, characterized in both their isotopic composition and impurity content. Three uncertainty budgets were evaluated, corresponding to three different cases involved in the measurements at the pilot laboratory: a) the temperature difference between two TPW cells, which amounted to 26 μK (k = 2), b) the temperature difference between a TPW cell and NRC national reference, which amounted to 35 μK (k = 2) and c) the temperature difference between a comparison participant’s transfer cell and the comparison reference, composed of the average of two NRC national reference cells, which amounted to 19 μK (k = 2).

No country for old borosilicate triple-point-of-water cells

“…Measurement setup. The new results presented in this paper were obtained using the measurement setup for the CCT-K7.2021 TPW key comparison which is described in detail in [8]. Briefly, for TPW resistance ratio measurements, we used a 25 Ω SPRT (Leeds and Northrup, s/n 1504288) and a 25 Ω standard resistor (Tinsley 5685A, s/n 274880) connected to a DC thermometry bridge (Measurements International MI6020T, s/n 1104110).…”

“…There was no uncertainty budget compiled for the measurements completed in 2007 and 2013, so one can use the NRC CCT-K7 (2002-2005) uncertainty (k = 2) of 29 μK (see table 18 in [9]) as a guiding value. The uncertainty (k = 2) in measuring the temperature difference between a TPW cell and NRC national reference in 2018 and 2021 was 46 μK [16] and 39 μK [8], respectively. Measurement method.…”

“…Measurement method. The new measurements of vitreoussilica TPW cells are described in [8]. The vitreous-silica TPW cells were split into two batches and the measurements took place in March-July, 2021.…”

“…Consequently, temperature metrology labs all around the world typically maintain a stock of several TPW cells which is checked on a regular basis [4][5][6][7]. At the national level, an ensemble of several carefully chosen TPW cells in a National Metrology Institute (NMI) forms the national standard for the TPW temperature [8]. The national standards are periodically intercompared between NMIs [9][10][11] usually using a transfer TPW cell specially selected for this purpose.…”

“…BS and Q indicate borosilicate and vitreous-silica TPW cells, respectively. The shaded bands represent the expanded uncertainty of 19 μK (k = 2)[8] for the temperature difference between a participant's transfer cell and the comparison reference.…”