The methods of realization of ITS-90 fixed points: the necessity for improvement

Abstract

The consistency of the results of international comparisons and the equivalence of national standards, improvement of methods for the realization of the International Temperature Scale ITS-90 main fixed points of metals freezing are considered. Two sources of uncertainty of the temperature value during realization of metals freezing fixed points have been determined. Experiments on the initiation of the inner interface between the liquid and solid phases of the metal have been carried out, the effect of the initiation conditions on the interface structure and measured temperature value in aluminum and indium fixed points cells has been investigated. The question of the correctness of the impurities influence estimates according to the laws for the only outer interface moving from the crucible walls to its center is considered. Analysis of the freezing curves obtained under different initiation conditions allowed determining the criterion for the presence of a continuous inner interface and the conditions for the formation of a single outer interface during slow crystallization of the metal without the formation of an inner interface. The obtained criteria will allow metrological institutes to choose the correct conditions for the initiation of the inner interface when using different designs of cells and heaters.