@staticmethod
def p1_hs(h, s):
    """function p1_hs = p1_hs(h, s)

        Supplementary Release on Backward Equations for Pressure as a Function of Enthalpy and Entropy p(h, s) to the IAPWS Industrial formulation 1997 for the Thermodynamic Properties of Water and Steam

        5 Backward Equation p(h, s) for Region 1

        Eqution 1, Table 2, Page 5
        """
    I1 = [0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 3, 4, 4, 5]
    J1 = [0, 1, 2, 4, 5, 6, 8, 14, 0, 1, 4, 6, 0, 1, 10, 4, 1, 4, 0]
    n1 = [-0.691997014660582, -18.361254878756, -9.28332409297335, 
        65.9639569909906, -16.2060388912024, 450.620017338667, 
        854.68067822417, 6075.23214001162, 32.6487682621856, -
        26.9408844582931, -319.9478483343, -928.35430704332, 
        30.3634537455249, -65.0540422444146, -4309.9131651613, -
        747.512324096068, 730.000345529245, 1142.84032569021, -436.407041874559
        ]
    eta = h / 3400
    Sigma = s / 7.6
    p = 0
    for i in range(0, 19):
        p = p + n1[i] * (eta + 0.05) ** I1[i] * (Sigma + 0.05) ** J1[i]
    return p * 100
