The Impact of Isotopic Concentration, Impurities, and Cell Aging on the Water Triple-Point Temperature

In 2005, the National Institutes of Standards and Technology (NIST) and Fluke’s Hart Scientific Division initiated a study to validate the isotopic correction algorithm applied to the realization temperature of triple point of water (TPW) cells. Additionally, the study quantified the impact of water sample impurities on the TPW cell realization temperature. For this study, eight TPW cells containing water of the same nominal isotopic concentration as Vienna Standard Mean Ocean Water (VSMOW) were used. Five of the cells were manufactured with fused-quartz envelopes and the remaining three with borosilicate envelopes. One TPW cell of each type was uniquely designed so that water samples could be periodically removed to analyze the isotopic composition and to monitor any changes in water purity with time and thereby correlate changes in composition with changes in realization temperature. The borosilicate TPW cells gave an average drift of −13 μK · yr−1 and the more stable fused-quartz TPW cells gave an average drift of −2 μK · yr−1.