code: def _Vapor_Density(cls, T):
          
          eq = cls._rhoG["eq"]
          Tita = 1-T/cls.Tc
          if eq == 4:
              Tita = Tita**(1./3)
          suma = 0
          for n, x in zip(cls._rhoG["ao"], cls._rhoG["exp"]):
              suma += n*Tita**x
          Pr = exp(suma)
          rho = Pr*cls.rhoc
          return rho
docs: Auxiliary equation for the density of saturated vapor
  
          Parameters
          ----------
          T : float
              Temperature, [K]
  
          Returns
          -------
          rho : float
              Saturated vapor density, [kg/m³]
  
          References
          ----------
          IAPWS, Revised Supplementary Release on Saturation Properties of
          Ordinary Water Substance September 1992,
          http://www.iapws.org/relguide/Supp-sat.html, Eq.3