The paper describes a modified form of the Benedict-Webb-Rubin thermal equation of state with 32 members received on the basis of its thermodynamic functions. The developed method of interpolation-analytical approximation of the modified Benedict-Webb-Rubin equation of state accounts for the real properties of working medium in 3D calculations. Constants of the equation of state are selected by on the basis of available thermodynamic property tables. On the one hand, it allows for sufficient accuracy, and on the other - does not require a significant increase in computational cost. The proposed method is validated using a sample working fluid - MDM (silica oil), which is used in ORC cycles of low-power cogeneration plants. It is shown that the mBWR32 equation with the obtained constants provides a good accuracy in the whole range of variation of thermodynamic values in the gaseous state and on the saturation line.