Realization of the triple point of carbon dioxide evaluated by the ITS–90

Abstract We performed a precise temperature measurement of the triple point of carbon dioxide (CO 2 ), which is one of the secondary reference points of the International Temperature Scale of 1990 (ITS–90), using an adiabatic calorimeter with five capsule-type standard platinum resistance thermometers (SPRTs). Impurity analyses showed that the purity of the CO 2 sample was 99.999 37%. A thermal treatment process before the realization of the triple point reduced the width of the melting curve of the triple point of CO 2 and increased the linearity of the melting curves with respect to the inverse of the melting fraction F . We determined the triple-point temperature of CO 2 at the liquidus point by extrapolating the six melting curves obtained after the thermal treatment process. The triple-point temperature of CO 2 measured in this work is 216.590 90 K 0.36 mK ( k = 1). This result indicates that the triple point of CO 2 can potentially be used as a fixed point based on the ITS–90 to replace the triple point of mercury.