@classmethod
def _Vapor_Entropy(cls, T):
    """Auxiliary equation for the specific entropy for saturated vapor

        Parameters
        ----------
        T : float
            Temperature, [K]

        Returns
        -------
        s : float
            Saturated liquid entropy, [kJ/kgK]

        References
        ----------
        IAPWS, Revised Supplementary Release on Saturation Properties of
        Ordinary Water Substance September 1992,
        http://www.iapws.org/relguide/Supp-sat.html, Eq.9
        """
    phi = cls._phi_sat(T)
    rho = cls._Vapor_Density(T)
    dpdT = cls._dPdT_sat(T)
    s = phi + dpdT / rho * 1000
    return s
