Within an international collaboration of the eight metrological institutes represented by the authors, the dependence of the triple-point temperature of equilibrium hydrogen on the deuterium content at low concentrations has been precisely determined so that the uncertainty in realizing the triple point as a temperature fixed point might be reduced by nearly one order of magnitude. To investigate the thermodynamic properties of the hydrogen-deuterium mixtures and to elucidate the factors that influence the melting temperature, 28 sealed fixed-point cells have been filled and measured, and some of these have been compared with an open-cell system. Hydrogen gas with a deuterium content ranging from 27.2 µmol D/mol H to 154.9 µmol D/mol H was studied using cells containing five different types of spin-conversion catalyst, with different catalyst-to-liquid volume ratios (a few per cent to more than 100%) and of different designs. The latter consideration is especially influential in determining the thermal behaviour of the cells and, thus, the temperature-measurement errors. The cells were measured at the eight participating institutes in accordance with a detailed protocol that facilitates a direct comparison of the results. Through analysis of the measurements, significant inter-institute deviations due to different measurement facilities and methods have been ruled out with respect to the determination of both the melting temperatures and the thermal parameters of the cells. The uncertainty estimates for the determination of the deuterium content have been verified by including isotopic analysis results from four different sources. The slope of the dependence of the triple-point temperature of equilibrium hydrogen isotopic mixtures on the deuterium content has been deduced from the melting temperatures of those sample portions not in direct contact with the catalysts. Evaluation of the data using different mathematical methods has yielded an average value of 5.4 2 µK per µmol D/mol H, with an upper bound of the standard uncertainty of 0.3 1 µK per µmol D/mol H. This is close to the literature value of 5.6 µK per µmol D/mol H that was obtained at higher deuterium concentrations.

Following the finalization of the work performed to establish the triplepoint temperature versus isotopic composition relationship for protium (Metrologia 42, 171 (2005)) adopted into the ITS-90 definition by the International Committee for Weights and Measures (CIPM) in 2005, and a preliminary exploration of the variability in the triple-point temperature of neon gas samples arising from differences in isotopic

The triple point of xenon is shown to be a suitable fixed point for incorporation into the next revision of the International Temperature Scale (ITS) as a means of reducing the non-uniqueness in the important 84 K to 273 K range. Isotopic effects, once thought to be a limiting factor, are shown to be negligible. Purity remains the overriding concern, but high-purity xenon gas is obtainable, giving a very flat plateau with a melting range within ±10 µK from 50% to 90% of the melted sample fraction. The triple point temperature of xenon is shown to be 161.405 96 K ± 0.32 mK (k = 1) on the ITS-90. The propagated calibration uncertainty of the platinum resistance thermometers and the component attributed to the non-uniqueness of the ITS-90, as evidenced by the differences among the seven calibrated thermometers, dominate the overall uncertainty of the estimated triple point temperature. The xenon triple point itself is highly reproducible, with a standard deviation of 48 µK for the eight melts of this study and a total realization uncertainty of just 76 µK.

Calibrated capsule-type standard platinum resistance thermometers were used to compare national realizations of the International Temperature Scale of 1990 (ITS-90) from 13.8033 K, the triple point of equilibrium hydrogen, to 273.16 K, the triple point of water, for seven countries in CIPM Key Comparison CCT-K2. Measurements were made at temperatures close to the eight low-temperature defining fixed points of the ITS-90, using a copper comparison block capable of simultaneously holding nine thermometers. Two separate measurement runs were performed, allowing two different groups of capsules from each laboratory to be examined. The results are used to determine the degree of equivalence of the independent national realizations of the scale for use in the Mutual Recognition Arrangement Appendix B database. In addition, measurements were made with the first group of thermometers at approximately eighty temperatures throughout the cryogenic range, which provide information to evaluate some of the so-called scale non-uniqueness issues inherent in the ITS-90 interpolation scheme.

The next revision to the International System of Units will emphasize the relationship between the base units (kilogram, metre, second, ampere, kelvin, candela and mole) and fundamental constants of nature (the speed of light, c, the Planck constant, h, the elementary charge, e, the Boltzmann constant, kB, the Avogadro constant, NA, etc). The redefinition cannot proceed without consistency between two complementary metrological approaches to measuring h: a ‘physics’ approach, using watt balances and the equivalence principle between electrical and mechanical force, and a ‘chemistry’ approach that can be viewed as determining the mass of a single atom of silicon. We report the first high precision physics and chemistry results that agree within 12 parts per billion: h (watt balance) = 6.626 070 63(43) × 10−34 J s and h(silicon) = 6.626 070 55(21) × 10−34 J s. When combined with values determined by other metrology laboratories, this work helps to constrain our knowledge of h to 20 parts per billion, moving us closer to a redefinition of the metric system used around the world.