def pressure_pseudocritical(ro_st):
    """
    Natural gas pseudocritical pressure (CH4 > 85%), MPa
    :param ro_st: (float) Natural gas density at standard parameters, kg/m3
    :return: (float) Pseudocritical pressure of the natural gas, MPa
    """
    return 0.1737 * (26.831 - ro_st)