def _PSat_T(T):
    """Define the saturated line, P=f(T)

    Parameters
    ----------
    T : float
        Temperature, [K]

    Returns
    -------
    P : float
        Pressure, [MPa]

    Notes
    ------
    Raise :class:`NotImplementedError` if input isn't in limit:

        * 273.15 ≤ T ≤ 647.096

    References
    ----------
    IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the
    Thermodynamic Properties of Water and Steam August 2007,
    http://www.iapws.org/relguide/IF97-Rev.html, Eq 30

    Examples
    --------
    >>> _PSat_T(500)
    2.63889776
    """
    # Check input parameters
    if T < 273.15 or T > Tc:
        raise NotImplementedError("Incoming out of bound")

    n = [0, 0.11670521452767E+04, -0.72421316703206E+06, -0.17073846940092E+02,
         0.12020824702470E+05, -0.32325550322333E+07, 0.14915108613530E+02,
         -0.48232657361591E+04, 0.40511340542057E+06, -0.23855557567849E+00,
         0.65017534844798E+03]
    tita = T+n[9]/(T-n[10])
    A = tita**2+n[1]*tita+n[2]
    B = n[3]*tita**2+n[4]*tita+n[5]
    C = n[6]*tita**2+n[7]*tita+n[8]
    return (2*C/(-B+(B**2-4*A*C)**0.5))**4