def d11_2_A11(d11, SRM_ratio=4.04367):
    """
    Convert Delta to Abundance notation.

    Default SRM_ratio is NIST951 11B/10B
    """
    return SRM_ratio * (d11 / 1000 + 1) / (SRM_ratio * (d11 / 1000 + 1) + 1)