Comparison of the scaling models for substance densities along saturation line

We discuss various thermodynamic equations used to represent the properties along the saturation line (fluid density, gas density, order parameter, mean diameter, etc) in a neighborhood of the critical temperature T_c. These properties are described scaling functions, depending on some parameters including the critical exponents α and β. Along with well-known models, we investigate a new model that represents the mean diameter as a sum of two scaling members in the critical region. The first term of this sum depends on the exponent, a and the second one depends on exponent, 2β. In the paper is given a methodological rationale for the new function representing the mean diameter. We have made numerical estimates for sulphur hexafluoride using the parameters involving with the above scaling equations.