The equation of state of water to 200°C and 3.5 GPa: model potentials and the experimental pressure scale

The velocity of sound in liquid water has been measured as a function of temperature and pressure in the heated diamond-anvil high-pressure cell. The equation of state is determined at temperatures up to 200°C and pressures as high as 3.5 GPa. Above 500 MPa, the results of molecular-dynamics simulations based on the TIP4P potential agree with the experimental density within 1%. Such simulations, however, do not achieve the accuracy required to usefully predict the thermodynamics of hydration reactions in the upper mantle. At pressures below 1 GPa a comparison is possible up to 150°C between velocities measured using the ruby fluorescence pressure scale and velocities based on an absolute pressure determination. Under the assumption that velocities deviations are a result of pressure uncertainty, the mean error of the ruby scale is 4 MPa (40 bars) with a standard deviation of 8 MPa (80 bars).