The temperatures of the triple points of methane and argon on IPTS-68

Abstract

The triple points of argon and methane were realised in a triple-point cell, adiabatically built in a flow cryostat of new design. The dependence of the triple-point temperature on the amount of the liquid phase was determined as a criterion of the purity of the gas used. For very pure argon (mole fraction of impurities less than 10−6), no measurable change of temperature could be detected in the range 25% to 90% of liquid phase. For methane (mole fraction of impurities 5×10−6), the temperature was constant between 15% and 90% of liquid phase. We found T68=83.7972 K for the temperature of the argon triple point and T68=90.6846 K for that of the methane triple point, on the International Practical Temperature Scale of 1968 (realisation of PTB), based on the oxygen condensation point. If the triple point of argon is used as a reference, with its assigned temperature T68=90.798 K, we find T68=90.6854 K.