code: def _Backward3_v_Ps(P, s):
      
      if s <= sc:
          return _Backward3a_v_Ps(P, s)
      else:
          return _Backward3b_v_Ps(P, s)
docs: Backward equation for region 3, v=f(P,s)
  
      Parameters
      ----------
      P : float
          Pressure, [MPa]
      s : float
          Specific entropy, [kJ/kgK]
  
      Returns
      -------
      v : float
          Specific volume, [m³/kg]