Thermodynamics of pure liquid water: Sound speed measurements to 700 MPa down to the freezing point, and an equation of state to 2300 MPa from 240 to 500 K

Abstract

Accurate thermodynamic properties for aqueous solutions under an increasing range of pressures, temperatures, and compositions are needed to address a variety of technical and scientific challenges. This study provides measurements for improving the high-pressure and low-temperature representation of liquid water. Sound speeds of pure liquid water are reported between 0.1 and 700 MPa, from 353 K down to the melting curves of the ice phases. The new sound speed measurements have a relative standard uncertainty of 100–170 ppm (from 0.1 to 700 MPa), with standard uncertainties of 20 mK for the temperature measurements and 0.02–0.14 MPa (from 0.1 to 700 MPa) for the pressure measurements. Using additional published measurements, a new equation of state is derived extending from 240 to 500 K and from 0.1 to 2300 MPa, covering much of the subcritical domain of water up to the ice VI–ice VII transition. Analyses of measurements and construction of the equation of state are accomplished with a flexible computational thermodynamic framework based on local basis functions in the form of tensor B-splines. Relative to IAPWS-95 (the most comprehensive representation available), improvements in the accuracies of density, sound speed, and specific heat are expected above 100 MPa, particularly near the solid-fluid phase boundaries.