Impurity Effect in Silver-Point Realization

Abstract

CCT-WG1 has recommended the sum of individual estimates (SIE) method to correct for the influence of impurities on the realization of temperature fixed points when a detailed impurity analysis is available. The method to estimate the uncertainty of the SIE has also been reported. On the other hand, most cells are fabricated from commercial fixed-point metals that often have no detailed impurity analysis, so the SIE calculation is impossible in that case. Due to this circumstance, and with the focus on the silver fixed point, a new fixed-point cell was fabricated in such a way that a portion of the silver ingot used was extractable during the silver casting. This portion was then analyzed by glow discharge mass spectrometry (GDMS), and the result used to calculate the SIE correction and its uncertainty. Temperature measurements during melting and freezing were collected using new and existing silver fixed-point cells under various conditions. These measurements were used to derive the slope of the silver freezing curve, from which the effect of impurities was evaluated by thermal analysis. The difference between the SIE and the thermal analysis method was evaluated to check the inaccuracy of the thermal analysis from the SIE point of view.