def _Backward3b_v_Ps(P, s):
    """Backward equation for region 3b, v=f(P,s)

    Parameters
    ----------
    P : float
        Pressure, [MPa]
    s : float
        Specific entropy, [kJ/kgK]

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

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for the
    Functions T(p,h), v(p,h) and T(p,s), v(p,s) for Region 3 of the IAPWS
    Industrial Formulation 1997 for the Thermodynamic Properties of Water and
    Steam, http://www.iapws.org/relguide/Supp-Tv%28ph,ps%293-2014.pdf, Eq 9

    Examples
    --------
    >>> _Backward3b_v_Ps(20,5)
    0.006262101987
    >>> _Backward3b_v_Ps(100,5)
    0.002449610757
    """
    I = [-12, -12, -12, -12, -12, -12, -10, -10, -10, -10, -8, -5, -5, -5, 
        -4, -4, -4, -4, -3, -2, -2, -2, -2, -2, -2, 0, 0, 0, 1, 1, 2]
    J = [0, 1, 2, 3, 5, 6, 0, 1, 2, 4, 0, 1, 2, 3, 0, 1, 2, 3, 1, 0, 1, 2, 
        3, 4, 12, 0, 1, 2, 0, 2, 2]
    n = [5.91599780322238e-05, -0.00185465997137856, 0.0104190510480013, 
        0.0059864730203859, -0.771391189901699, 1.72549765557036, -
        0.000467076079846526, 0.0134533823384439, -0.0808094336805495, 
        0.508139374365767, 0.00128584643361683, -1.63899353915435, 
        5.86938199318063, -2.92466667918613, -0.00614076301499537, 
        5.76199014049172, -12.1613320606788, 1.67637540957944, -
        7.44135838773463, 0.0378168091437659, 4.01432203027688, 
        16.0279837479185, 3.17848779347728, -3.58362310304853, -
        1159952.60446827, 0.199256573577909, -0.122270624794624, -
        19.1449143716586, -0.0150448002905284, 14.6407900162154, -
        3.2747778718823]
    Pr = P / 100
    sigma = s / 5.3
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (Pr + 0.298) ** i * (sigma - 0.816) ** j
    return 0.0088 * suma
