From CCT-K7 to CCT-K7.2021: Approaching the definition of the triple point of water temperature

• Main Authors: Dedyulin, S., Peruzzi, A., del Campo, D., Izquierdo, B. C. Garcia, Gomez, M. E., Quelhas, K. N., Neto, M. A. P., Lozano, B. M., Eusebio, L., Yang, I., Sparasci, F., Martin, C., Risegari, L., Saunders, P., Molloy, E., Yan, X. K., Sun, J. P., Feng, X. J., Zhang, J. T., Ho, M.-K., Nakano, T., Widiatmo, J. V., Saito, I., Ejigu, E., Pearce, J., Rusby, R., Rudtsch, S., Buenger, L., Kalemci, M., Uytun, A., Bruin-Barendregt, C., Panman, M., van Geel, J., White, D. R., Possolo, A.
• Format: Conference Proceeding
• Language: English
• Subjects: Seawater; Skewed distributions; Temperature scales; Water temperature
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/5.0234488

In 2002–2004, the first CIPM Key Comparison of triple-point-of-water (TPW) cells, CCT-K7, was carried out. One of the major observations in CCT-K7 was the bimodal distribution of the results because 3 out of 21 participating labs had their TPW realization based on Vienna Standard Mean Ocean Water (V-SMOW) and thus followed the definition of the TPW given in the Supplementary Information to the International Temperature Scale of 1990 (ITS-90), which resulted in “warmer” national realizations compared to the rest of the participants. Based on the CCT-K7 results, the “Clarification of the definition of the kelvin” was issued in 2005, where the importance of the isotopic corrections to V-SMOW water was emphasized. The successor of CCT-K7, CCT-K7.2021, was carried out in 2021–2022. In this paper, we compare the results of the two comparisons side-by-side to illustrate: a) the improved measurement capabilities of the participants over the past 20 years and b) the improved quality of the TPW cells. As expected, in CCT-K7.2021 most participants renewed their national reference ensembles with newer, higher quality and vitreous-silica cells and applied the isotopic corrections to their respective national references. This, in turn, led to: 1) a smaller spread of the national realizations by almost a factor of two — 28 µK instead of 50 µK in CCT-K7, 2) an only slightly asymmetric distribution of the results, and 3) an increase in KCRV value by approximately 66 µK compared to CCT-K7.