The Long-Term Drift of Triple-Point-of-Water Cells

Abstract

As the triple point of water is of great importance for the International Temperature Scale of 1990 (ITS-90) and for the definition of the unit of thermodynamic temperature, its long-term stability has attracted a great deal of attention. In a study of long-term stability, a mystery has been uncovered. Some triple-point-ofwater cells remain stable for many decades, while others decrease with increasing age of the cells, which is called long-term drift. To investigate this mystery, we used cells with different manufacture dates ranging from 1974 to 2002 and compared their analyses, which were done in 1984 and 2003. Using the same model of long-term drift as that used by Hill, the long-term drift rates of the two data sets are −4.7 µK · year −1 and −9.2 µK · year −1 , respectively. One is consistent with the observed depression of about −4 µK · year −1 measured by Hill, whereas the other differs greatly from Hill's result. In addition, corresponding factors influencing long-term drift are discussed in this paper.Keywords International Temperature Scale of 1990 (ITS-90) · Long-term drift · Residual air · Solubility of borosilicate glass · Triple point of water

Improved electronic measurement of the Boltzmann constant by Johnson noise thermometry

“…However, there are errors and uncertainties associated with the practical realization of the triple point, all of which are summarised in detail in [55]. Because the cell was manufactured without an isotopic analysis, and chemical analysis would require the destruction of the cell, the temperature realized in the cell was determined by comparing it to the NIM standard TPW cell [56,57] with a long-stem standard platinum resistance thermometer (SPRT). The temperature realized in the cell was found to be 1.46 mK higher than that in the NIM standard cell.…”

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

“…In addition, since gaseous impurities also tend to have some finite solubility in water, they may affect the realized TPW temperature in the same manner as the impurities present in the water due to dissolution of the glass. It was reported during regular TPW intercomparisons [4][5][6] that accumulation of residual gases in several sealed TPW cells decreased the realized TPW temperature by more than 100 μK.…”

“…Thus, throughout the temperature measurements, a TPW cell is used multiple times in order to: (a) assess the stability of the SPRT as part of a regular check-up procedure and (b) calculate W(T 90 ) using TPW measurements made immediately after the measurement at T 90 (or made with the SPRT in the same oxidation state) [3]. 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].…”