The Comparison of MTDATA with the Melting/Freezing Point Curves of ITS-90 Metal Fixed Points

The International Temperature Scale (ITS-90) is defined in part by a series of metal freezing points between 156 °C and 1,084 °C. These freezing-point cells provide reference temperatures with an uncertainty of realization claimed to be in the range of several tenths of a millikelvin. The impurities in the nominally 99.9999 % pure metals make a major contribution to the uncertainty of realization of the fixed-point temperature. Recently, a new method to correct for the influence of the impurities by summing the individual contributions of each impurity has been suggested. This method is referred to as the “sum of individual estimates” (SIE). NPL is a partner in a Euromet project to improve the realization of ITS-90 metal fixed points. As part of this project, NPL is particularly interested in the fixed-point cells of tin (231.928 °C) and aluminum (660.323 °C). This article describes the use of a thermodynamic model, embodied in NPL’s MTDATA software, to estimate the initial drop in the freezing temperature and the temperature decrease during freezing using both equilibrium and “Scheil” approaches. Calculations of this type establish the effect of single elements, and with the chemical analysis of the metal, enable an estimation of the whole freezing curve. This has been done for a sample of tin from this laboratory, and thereafter, the theoretical curves are compared with previously published experimental data on impurity-doped aluminum, with good agreement, e.g., better than 1 mK over most of the curve for 76 μg · g−1 Ag in Al.