The evaluation of impurity effect on the realization of fixed-point temperatures of the International Temperature Scale of 1990, by which the temperature standard is defined, is essential, since impurity in many cases lowers the fixed-point temperature.

First, the impurity effect was evaluated by a theoretical approach using the impurity information obtained from a chemical analysis of the fixed-point substance. A direct comparison of fixed-point temperatures realized by two different substances was conducted as the second approach. The applicability of these two methods, however, is limited so that the impurity effect cannot be evaluated properly. To this concern, analysis on the changes of temperature and liquid fraction during the realization of the fixed point was performed as the third method. It has been found here that this third method provides a lot of accurate information for estimating the defined temperature, including those provided by the other two methods. Based on the third method, a world top class temperature standard has been realized.