def surface_margin_deph (A_approx_deph, A_real_deph):
    """
    Calculates the surface margin.
    Parameters
    ----------
    A_approximate_deph : float
        The approximate heat ransfer area, [m**2]
    A_real_deph : float
        The real heat transfer area, [m**2]
    Returns
    -------
    surface_margin_deph : float
        The surface margin [%]
    References
    ----------
    &&&&
    """          
    return (A_approx_deph - A_real_deph) * 100 / A_approx_deph