Speed of sound of pure water to 700 MPa and an equation of state to 2300 MPa

Abstract

Sound speeds of pure fluid water are reported between 0.1 and 700 MPa, from 353 K down to the melting curves of the ice phases. The 2 sigma precision and accuracy of the new sound speed measurements are close to 0.02%, with an estimated pressure accuracy of 0.04% up to 700 MPa. 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 sub-critical domain of water up to the ice VI - ice VII transition. Analysis 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 observed above 100 MPa, particularly near the solid-fluid phase boundaries. 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: the present study provides measurements essential for improving the high-pressure and low temperature representation of water.