def _D2O_Tension(T):
    """Equation for the surface tension of heavy water

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

    Returns
    -------
    sigma : float
        Surface tension [N/m]

    Raises
    ------
    NotImplementedError : If input isn't in limit
        * 269.65 ≤ T ≤ 643.847

    Examples
    --------
    >>> _D2O_Tension(298.15)
    0.07186
    >>> _D2O_Tension(573.15)
    0.01399

    References
    ----------
    IAPWS, Release on Surface Tension of Heavy Water Substance,
    http://www.iapws.org/relguide/surfd2o.pdf
    """
    Tr = T/643.847
    if 269.65 <= T < 643.847:
        return 1e-3*(238*(1-Tr)**1.25*(1-0.639*(1-Tr)))
    else:
        raise NotImplementedError("Incoming out of bound")