Isochoric Heat Capacity Measurements for Light and Heavy Water Near the Critical Point

The isochoric heat capacity was measured for D2O at a fixed density of 356.075 kg } m&3 and for H2O at 309.905 kg } m&3. The measurements cover the range of temperatures from 623 to 661 K. The measurements were made with a high-temperature, high-pressure, adiabatic calorimeter with a nearly constant inner volume. The uncertainty of the temperature is 10 mK, while the uncer- tainty of the heat capacity is estimated to be 2 to 30. Measurements were made in both the two-phase and the one-phase regions. The calorimeter instrumenta- tion also enables measurements of PVT and the temperature derivative ( P T)V along each measured isochore. A detailed discussion is preseInted on the experimental temperature behavior of CV in the one- and two-phase regions, including the coexistence curve near the critical point. A quasi-static thermo- gram method was applied to determine values of temperature at saturation TS(\) on measured isochores. The uncertainty of the phase-transition tem- perature measurements is about \0.02 K. The measured CV data for D2O and H2O are compared with values predicted from a recent developed parametric crossover equation of state and IAPWS-95 formulation.