Isotopic and other influences on the realization of the triple point of hydrogen

Abstract

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.

Measuring Systems for Thermometer Calibration in Low-Temperature Range

“…5. These results were confirmed by other partners realizing the project [17][18][19]. Table 2 the calibration and measurement services of INTiBS in the low-temperature range are presented.…”

Cryocrystal phase transitions applied as temperature standards

“…11. The isotopic composition of natural hydrogen varies from 25 to 185 µmol D/mol H. In a frame of an international cooperation several hydrogen cells with a different isotopic composition were tested [12,13]. Experimental results, confirming the theory, are presented in Fig.…”

“…Basing on these results, recommendation of the CCT has been made to add the information to a definition of the hydrogen triple-point temperature in the ITS-90 [14].…”

The ITS-90 after definition of neon isotopic reference composition: extent of the isotopic effect tested on previous inter-comparison results

“…Isotopic studies were undertaken, initially on e-H 2 (HD in H 2 ) [5,6] and H 2 O (D 2 and 18 O 2 ) [7][8][9][10], and later also on Ne [4,[11][12][13][14][15].…”