We have determined the acoustic and microwave frequencies of a misaligned spherical resonator maintained near the temperature of the triple point of water and filled with helium with carefully characterized molar mass M = (4.002 6032 ± 0.000 0015) g mol

The triple point of water serves to define the kelvin, the unit of thermodynamic temperature, in the International System of Units (SI). Furthermore, it is the most important fixed point of the International Temperature Scale of 1990 (ITS-90). Any uncertainty in the realization of the triple point of water contributes directly to the measurement uncertainty over the wide temperature range from 13.8033 K to 1234.93 K.The Consultative Committee for Thermometry (CCT) decided at its 21st meeting in 2001 to carry out a comparison of water triple point cells and charged the BIPM with its organization.Water triple point cells from 20 national metrology institutes were carried to the BIPM and were compared with highest accuracy with two reference cells. The small day-to-day changes of the reference cells were determined by a least-squares technique. Prior to the measurements at the BIPM, the transfer cells were compared with the corresponding national references and therefore also allow comparison of the national references of the water triple point.This report presents the results of this comparison and gives detailed information about the measurements made at the BIPM and in the participating laboratories. It was found that the transfer cells show a standard deviation of 50 µK; the difference between the extremes is 160 µK. The same spread is observed between the national references.The most important result of this work is that a correlation between the isotopic composition of the cell water and the triple point temperature was observed. To reduce the spread between different realizations, it is therefore proposed that the definition of the kelvin should refer to water of a specified isotopic composition.The CCT recommended to the International Committee of Weights and Measures (CIPM) to clarify the definition of the kelvin in the SI brochure by explicitly referring to water with the isotopic composition of Vienna Standard Mean Ocean Water (VSMOW). The CIPM accepted this recommendation and the next edition of the SI brochure will include this specification.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

We report speed of sound measurements in helium at 273.16 K and at eight temperatures in the range between 236 K and 430 K. These results determine the difference (T  −  T 90) between the thermodynamic temperature T and its approximation T 90 by the International Temperature Scale of 1990 (ITS-90). The uncertainty of our measurements of (T  −  T 90) spans between a minimum of 0.25 mK near 247 K and a maximum of 0.89 mK at the freezing point of indium (429.75 K) with comparable contributions from the uncertainty of our acoustic determination of T and from the uncertainty of our laboratory realization of ITS-90. On the overlapping temperature ranges these results are consistent with other recent acoustic determinations of (T  −  T 90). We also present evidence that (T  −  T 90) can be determined with comparably small uncertainties by the alternative, time-saving procedure of measuring the speed-of-sound in helium using only a single, judiciously-chosen, pressure on each isotherm.

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