The Comparison Between the Freezing Point and Melting Point of Tin

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The Comparison Between the Freezing Point and Melting Point of Tin (43.8 KB)

Preliminary data from thirteen Standard Platinum Resistance Thermometers (SPRTs) on eighteen pair of freezing and melting curves of high-purity tin (better than 99.9999%) showed that the average calibration difference between the melting point and freezing point was within approximately 0.3 mK. Therefore, the melting point of tin (MPT) can be used to calibrate SPRTs instead of the freezing point of tin (FPT) for most situations.

Further investigation was made toward improvement of the reproducibility and accuracy of MPT in order to see if it is possible to use it instead of FPT as a defining fixed point for the International Temperature Scale as is the case with gallium. In order to acquire the highest accuracy from MPT, the following key points were found to be vital: the highest possible purity tin must be used; the temperature uniformity in the furnace should be better than 10 mK throughout a vertical distance of 180 mm; an inner melting technique must be used to create two liquid-solid interfaces in the crucible during the entire melting course; and in order to distribute all remaining impurities uniformly, the tin should be maintained overnight at a temperature of 1°C below the melting point. Two SPRTs were calibrated in this way at the MPT and the FPT alternately. The average differences between the melting point and the freezing point were well within 0.1 mK for both SPRTs. It seems that the MPT, rather than the FPT, may be used as a defining fixed point of the International Temperature Scale.