Development of the Freezing Point of Tin in the ITS-90

Abstract

A new apparatus consisting of a fixed point cell and a furnace was developed to evolve the accuracy of realizing the freezing point of tin (231.928°C) defined in the International Temperature Scale of 1990 (ITS-90). The temperature stability and uniformity of the furnace are better than a few millikelvins. This apparatus can realize the freezing plateaux lasting longer than 45 hours, with a slight temperature drop within 0.3mK. From this prolonged phase transition, the freezing process will come closer to the ideal freezing point under the thermodynamic equilibrium. Since the tin point shows a large supercooling in the freezing process compared to the other freezing points, a newly designed gas-cooling instrument was introduced for safe operation and precise cooling control. Such the apparatus and the optimized procedure give satisfactory repeatability within 0.026mK from ten realizations of the freezing point. These results confirm the capability of the new apparatus for replacing the current one as the national standard.