Comparison of triple point of water cells with isotopic composition between NIM and PTB

The triple point of water (TPW) is the most important defining fixed point on the International Temperature Scale of 1990 (ITS-90), because the uncertainty in the realization of the TPW is directly propagated to the ITS-90 over the temperature range from 13.8033 K to 1234.93 K. The triple point temperature of water depends significantly on the isotopic composition and impurity contents of water in the TPW cells. In the CCT-K7 key comparison of water triple point cells, the temperature differences of the national references were mainly attributed to the isotopic composition. In order to verify the international equivalence of TPW based on the Vienna Standard Mean Ocean Water (V-SMOW), a bilateral comparison of TPW cells with isotopic composition was carried out between NIM and PTB. After application of the corrections for isotopic composition, national realizations of the triple point temperature of water in China and Germany agree to within 0.0005 mK with an uncertainty of 0.03 mK ( k = 1), indicating good equivalence. Additionally, with a value of 0.008 mK of agreement between NIM and PTB, the new CCT-K7.2021 key comparison results confirm this level. Incidentally, an electronic immersion cooler was used to prepare the ice mantles at PTB. After 12 hours when the ice mantles were formed, the TPW temperature was stabilized within 0.02 mK.