def _Backward3_v_Ph(P, h):
    """Backward equation for region 3, v=f(P,h)

    Parameters
    ----------
    P : float
        Pressure, [MPa]
    h : float
        Specific enthalpy, [kJ/kg]

    Returns
    -------
    v : float
        Specific volume, [m³/kg]
    """
    hf = _h_3ab(P)
    if h <= hf:
        return _Backward3a_v_Ph(P, h)
    else:
        return _Backward3b_v_Ph(P, h)
Backward equation for region 3, v=f(P,h)

    Parameters
    ----------
    P : float
        Pressure, [MPa]
    h : float
        Specific enthalpy, [kJ/kg]

    Returns
    -------
    v : float
        Specific volume, [m³/kg]