Accuracy of Approximations to the Poynting Correction for Ice and Liquid Water

Abstract

Rigorous calculation of the Poynting correction, which describes the effect of pressure on the fugacity of a condensed phase, requires time-consuming evaluation of a thermodynamic potential such as the International Association for the Properties of Water and Steam (IAPWS-95) formulation for water. Simplifying approximations are used in many applications, but the error introduced by the approximations is seldom evaluated. In this work, first-order and second-order approximations were developed for the Poynting correction for both ice and liquid water (including supercooled liquid water), and their errors were evaluated by comparison to the full thermodynamic potentials. The range of conditions covered is from −100 °C to 200 °C at pressures to 20 MPa. Some implications for the calculation of the enhancement factor used in humidity metrology are discussed.