Calculation of $\rm SF_6$ thermodynamic properties, including a critical region. Combined thermal equation of state with a small number of parameters

A new unified combined equation of the state of liquid, gas, and fluid with a small number of adjustable constants is considered. Using this equation, tables of sulfur hexafluoride $\rm SF_6$ thermodynamic properties are compiled and proposed as standard reference data. The equation of state has the form of an explicit dependence of $p = p(\rho, T)$ and does the following: transfers the thermal and caloric properties of $\rm SF_6$ in the regular component of the thermodynamic surface with the accuracy of the experiment; includes critical exponents $\beta$, $\alpha$, and $\gamma$, taken in agreement with the three-dimensional Ising model; and corresponds to the relations of thermodynamics and the scale theory for properties in the critical region. The adjustable constants are obtained by approximating the $p, \rho, T$-data of $\rm SF_6$ using a combined equation of state in the ranges of $224 \le T \le 623$ K, $0.1 \le p \le 30$ MPa, and $8 \le \rho \le 1842$ kg/m$^3$. It includes a new regular component for approximating $p, \rho, T$-data in the liquid and gas regions of states outside the critical region, four of the $13$ constants that are determined by the conditions at the critical and triple points, the singular component that is a scale equation (six constants) in explicit form for the critical region, and a new crossover function for combining these equations. The total number of system-dependent constants is $21$, including parameters of the critical point. The root-mean-square error in the description of $p, \rho, T$-data of $\rm SF_6$ by pressure is $1.05\%$ in the entire range of gas and liquid states; the average absolute deviation is $0.51\%$.