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

Abstract

Temperature calibration labs all around the world typically maintain a set of several triple-point-of-water (TPW) cells for regular measurements using the International Temperature Scale of 1990 (ITS-90). This set often includes more-than- 10-year-old TPW cells made from a borosilicate glass. In this paper we summarize the problems with old borosilicate TPW cells, using the available literature data, the results of CCT.K7-2021 key comparison and NRC TPW measurements, and highlight the solution to these issues: vitreous-silica TPW cells. The latter have an exceptional long-term stability, as we demonstrate by using: a) comparative measurements of the same-age, same-manufacturer vitreous-silica and borosilicate TPW cells at NRC in a period from 2007 to 2021, and b) Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analysis of impurities present in the 18 year-old vitreous-silica outlier, cell Q325. Remarkably, not only all NRC vitreous-silica TPW cells remained stable over a 15-year time period, unlike their borosilicate counterparts, but the amount of impurities in the "coldest" cell Q325 (-31 microK from the intercomparison reference value) is comparable to that in the newly purchased vitreous-silica cells. We argue that the most accurate TPW measurements, such as for defining the national TPW references and for the international key comparisons, should rely exclusively on vitreous-silica TPW cells.