Stefan Herrig, Monika Thol, Allan H. Harvey, Eric W. Lemmon
An empirical fundamental equation of state is presented for heavy water (deuterium oxide, D2O). The equation is explicit in the reduced Helmholtz energy and allows the calculation of all thermodynamic properties over the whole fluid surface. It is valid from the melting-pressure curve up to a temperature of 825 K at pressures up to 1200 MPa. Overall, the formulation represents the most accurate measured values and almost all the other available data within their experimental uncertainty. In the homogenous liquid and vapor phase, the expanded relative uncertainties of densities calculated from the equation of state are mostly within 0.1 % or less; liquid-phase densities at atmospheric pressure can be calculated with an uncertainty of 0.01 %. The speed of sound in the liquid phase is described with a maximum uncertainty of 0.1 %; the most accurate experimental sound speeds are represented within their uncertainties ranging from 0.015 % to 0.02 %. In a large part of the liquid region, the isobaric heat capacity is represented with an uncertainty of 1 %. The uncertainty in vapor pressure is mostly within 0.05 %. In the critical region, the uncertainties of calculated properties are in most cases higher than the values given above. Nevertheless, the equation of state enables a reasonable description of this region. The equation matches available data for the metastable subcooled liquid, and it extrapolates in a qualitatively correct way to extreme values of temperature and pressure. This formulation is the result of an effort to produce a new standard for the thermodynamic properties of heavy water by the International Association for the Properties of Water and Steam (IAPWS).