MEASUREMENTS OF THE PRESSURE-VOLUME-TEMPERATURE PROPERTIES OF FLUIDS TO 20 KBAR AND 1000°C: A NEW APPROACH DEMONSTRATED ON H2O

Abstract

A new method for measuring volumes of fluids in a piston cylinder apparatus has been tested and applied to measuring the molar volume of H2O VH2O from 8.5 to 20 kbar and 800° to 1000°C. A thick-walled nickel capsule partially filled with H2O is run at the desired pressure and temperature, allowing it to deform until pressures inside and outside the capsule equate. Rapidly quenching the experiment preserves the capsule volume, which is measured by weighing the capsule in air and in H2O. Subtracting the known volume of the metal in the capsule from the total capsule volume yields the inner volume of the capsule, which represents the equilibrium volume of the fluid at pressure and temperature. Multiple experiments defined the conditions under which capsules are strong enough to resist deformation on the short timescale of the quenching process, but weak enough to compact to achieve an equilibrium volume during the longer timescale of the experiment. The time required for the capsule to attain an equilibrium volume is between one and two days at 8.5 kbar and 800°C and 180 and 460 min at 17.5 kbar and 1000°C. Errors arising from modification of capsule volume during quenching appear minor at 10 kbar for temperatures up to 1000°C, but become significant at 15 kbar for temperatures above 900°C, resulting in poor precision or underestimation of the equilibrium volume. The total relative error in molar volume arising from uncertainties in pressure, temperature, and volume measurement is #2%. Measurements overlapping with those of Burnham et al. (1969) are in good agreement, suggesting the method is accurate.The equation of Brodholt and Wood (1993) fits our data best at pressures >10 kbar but is not accurate at pressures <10 kbar and should be used only in combination with another equation of state accurate between zero 10 to estimate the values integrated thermodynamic quantities. fugacity h2O from 10-30 kbar and 600-1200°C calculated by combining the equations of state of Kerrick and Jacobs (1981, 1 bar-10 kbar) and Brodholt and Wood (1993, >10 kbar) are presented and fit with a polynomial to allow easy calculation of fugacity.