Thermodynamic properties of fluid helium at high pressures determined from speeds of sound

The equation of state and heat capacity of fluid helium have been determined at temperatures from 98.15 to 298.15 K and at pressures from 100 to 1000 MPa using a new computational method. This is based on stepwise construction of ρT isobars starting from an experimental reference pρ isotherm and using the previously reported sound speed data of helium as a function of pressure and temperature. In the region of overlap, the calculated equation-of-state data show significant discrepancies from those obtained previously with direct measurements.