Residual resistance ratio as indicator for the influence of impurities on fixed-point temperatures

Abstract

Abstract The component caused by the influence of impurities often dominates the uncertainty budgets of realisations of temperature fixed points. The Consultative Committee for Thermometry recommended, therefore, in section 2.1 Fixed Points: Influence of Impurities of the Guide to the Realization of the ITS-90 (www.bipm.org) two methods for estimating this component. One is based on summing up the individual effects of all relevant impurities. This method causes huge efforts. The second one requires a reliable information on the overall impurity concentration, which is usually not supplied by the producers of the fixed-point materials. In the paper, it is analysed if the determination of the residual resistance ratio r 0 can aid in getting the required information [r 0 means the ratio of the electrical resistivities ρ at a sufficiently low temperature and at 293 K, usually r 0 = ρ(4.2 K)/ρ(293 K)]. The crucial criterion is the sensitivity of r 0 for the influence of an impurity on the fixed-point temperature, i.e. the magnitude of the derivative dT l/dr 0 of the liquidus-line temperature T l with respect to r 0. As examples, the developed methodology is applied to three fixed points of the ITS-90, namely the freezing points of indium, zinc, and aluminium. It can be shown that the use of r 0 can reduce the number of impurities, for which the effortful determination of the individual concentrations is necessary, by an order of magnitude, namely to about 10 or below.