Calorimetric determination of the deviation between the International Practical Temperature Scale 1968 and the Thermodynamic Temperature Scale

Abstract

A flow calorimeter was constructed to measure the isobaric heat capacity cp for dilute fluids for pressure [Formula: see text] and for temperature [Formula: see text]. The special design features of the calorimeter facilitate measurements at extremely high accuracy. In this paper the measurements have been used to explore the metrology of the temperature. Measured values of cp were compared with corresponding theoretical values calculated via statistical mechanics. From the comparison, the deviation between the International Practical Temperature Scale of 1968 and the (theoretical) Thermodynamic Temperature Scale has been obtained. The results of this metrology are in good agreement with those obtained with the PVT gas thermometer and those obtained by radiometry. The comparison establishes the accuracy of the calorimeter and, more important, the feasibility of cp metrology, especially for probing local stretching or contraction of the International Practical Temperature Scale. The integration of this distortion, starting from the triple point of water, yields the deviation between the scales. It is, e.g., between 30 mK and 40 mK at the boiling point of water, according to our results.