The Span-Wagner (non-polar and polar) equations of state are described in:
Span, R.; Wagner, W. - Equations of State for Technical Applications. I. Simultaneously Optimized Functional Forms for Nonpolar and Polar Fluids. Int. J. Thermophys. 2003, 24(1), 1-39.
The specific form of the equation used in TDE is given on page 71 of the article (page 611 of the Journal) describing the IUPAC ThermoML data-communication standard.
A link to the article is here: Pure Appl. Chem., Vol. 78, No. 3, pp. 541-612, 2006.>
Equation parameters are available in several formats from TDE: Equation Parameters in TDE