code: def _Backward3_sat_v_P(P, T, x):
      
      if x == 0:
          if P < 19.00881189:
              region = "c"
          elif P < 21.0434:
              region = "s"
          elif P < 21.9316:
              region = "u"
          else:
              region = "y"
      else:
          if P < 20.5:
              region = "t"
          elif P < 21.0434:
              region = "r"
          elif P < 21.9009:
              region = "x"
          else:
              region = "z"
  
      return _Backward3x_v_PT(T, P, region)
docs: Backward equation for region 3 for saturated state, vs=f(P,x)
  
      Parameters
      ----------
      T : float
          Temperature, [K]
      P : float
          Pressure, [MPa]
      x : integer
          Vapor quality, [-]
  
      Returns
      -------
      v : float
          Specific volume, [m³/kg]
  
      Notes
      -----
      The vapor quality (x) can be 0 (saturated liquid) or 1 (saturated vapour)