def _Tension(T):
    """Equation for the surface tension

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

    Returns
    -------
    σ : float
        Surface tension, [N/m]

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

        * 248.15 ≤ T ≤ 647
        * Estrapolate to -25ºC in supercooled liquid metastable state

    Examples
    --------
    >>> _Tension(300)
    0.0716859625
    >>> _Tension(450)
    0.0428914992

    References
    ----------
    IAPWS, Revised Release on Surface Tension of Ordinary Water Substance
    June 2014, http://www.iapws.org/relguide/Surf-H2O.html
    """
    if 248.15 <= T <= Tc:
        Tr = T/Tc
        return 1e-3*(235.8*(1-Tr)**1.256*(1-0.625*(1-Tr)))
    else:
        raise NotImplementedError("Incoming out of bound")