A Reference Equation of State for the Thermodynamic Properties of Sulfur Hexafluoride (SF₆) for Temperatures from the Melting Line to 625  K and Pressures up to 150  MPa

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A new equation of state for the thermodynamic properties of the fluid phase of sulfur hexafluoride (SF₆) in the form of a fundamental equation explicit in the Helmholtz energy is presented. The functional form consists of a part describing the ideal-gas state and the residual part as the difference between the real-fluid and the ideal-gas behavior. The residual part was developed using state-of-the-art linear and nonlinear optimization algorithms. It contains 36 coefficients, which were fitted to selected data for the thermal and caloric properties of sulfur hexafluoride in the single-phase region and on the vapor-liquid phase boundary. Especially for the thermal properties in the critical region, a very extensive and high-precision data set was available. In this work, information on the experimental data for the thermodynamic properties and all details of the new equation are presented. The new equation of state describes the pT surface of sulfur hexafluoride with an uncertainty in density of less than 0.02%–0.03% from the melting line up to temperatures of 500  K and pressures of 30  MPa. In the critical region, including the immediate vicinity of the critical point, the uncertainty in pressure is less than 0.01%. Reliable data sets of other thermodynamic properties are reproduced within their experimental uncertainties. The primary data, to which the equation was fitted, cover the fluid region from the melting line to temperatures of 625  K and pressures up to 150  MPa. Beyond this range, the equation can be extrapolated with a physically reasonable behavior up to very high temperatures and pressures. In addition to the equation of state, independent equations for the vapor pressure, the saturated-liquid and saturated-vapor densities, the melting pressure, and the sublimation pressure are given. Tables of thermodynamic properties calculated from the new equation of state are listed in the Appendix. ©2009 American Institute of Physics