Assessment of fused-silica triple point of water cells developed at NIM

Due to the gradual dissolution of glass components by leaching and etching processes, impurities from borosilicate glass containers dissolve into water, causing an increasing impurity concentration in the water of triple point of water (TPW) cells. As a result, increasing impurity-induced temperature depressions with ages are observed in old borosilicate TPW cells. Thanks to the extraordinary physical-chemical properties of fused silica glass, especially the outstanding resistance to dissolution in water, fused-silica cells have a very much lower long-term drift rate than borosilicate cells. In recent years, many efforts have been devoted to developing fused-silica cells and evaluating their performance. Due to the liberation of gaseous components and permeability of silica glass to helium and other gases at high temperatures during the sealing of fused silica cells with a hydrogen-oxygen flame, it is very challenging to limit the production of residual gases and prevent the dissolution in the water. For this reason, fused-silica cells are thought to have higher initial impurity concentration than borosilicate cells. To assess the performance of fused-silica cells, four newly fabricated borosilicate cells were chosen as reference cells and compared with seven fused silica cells. The results, after correction for the effects of varying isotopic composition, hydrostatic pressure, and self-heating, indicated negligible difference between borosilicate and fused-silica cells. The quality of the NIM cells and cell fabrication processes is supported by the smallest temperature difference of just −1.1 μK between the NIM national reference and the key comparison reference value (KCRV) of CCT-K7.2021.