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 section2.1 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 293K, usually r ₀ = ρ(4.2K)/ρ(293K)]. 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.

由杂质影响引起的分量通常支配着温度固定点实现的不确定性预算。因此，温度测量咨询委员会在 ITS-90 实现指南(www.bipm.org) 的第 2.1节“固定点：杂质的影响”中推荐了两种估算该成分的方法。一种是基于总结所有相关杂质的个别影响。这种方法需要付出巨大的努力。第二个需要关于总体杂质浓度的可靠信息，而定点材料的生产商通常不提供这些信息。文中分析了残余电阻比r ₀的确定可以帮助获得所需的信息 [ r ₀表示在足够低的温度和 293K 下的电阻率ρ的比率，通常r ₀ = ρ (4.2K)/ ρ (293K)]。关键标准是r ₀对杂质对定点温度影响的灵敏度，即液相线温度T _l的导数 d T _l /d r ₀相对于r _{0 的大小} . 例如，将开发的方法应用于 ITS-90 的三个固定点，即铟、锌和铝的凝固点。可以表明，使用r ₀可以将杂质的数量减少一个数量级，即减少到大约10或更低，对于这些杂质，需要费力地确定各个浓度。