def _Backward3_v_Ps(P, s):
    """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]
    """
    if s <= sc:
        return _Backward3a_v_Ps(P, s)
    else:
        return _Backward3b_v_Ps(P, s)