def mangle_length_unit_ascii(unit):
    """
    Convert unit string to ASCII representation, e.g. converts 'µm'
    to 'um'.

    Parameters
    ----------
    unit : str
        Name of unit

    Returns
    -------
    output_unit : str
        Mangled name of unit
    """
    unit = unit.strip()
    if unit == '':
        return None
    elif unit == 'Å':
        return 'A'
    elif unit == 'μm' or unit == 'µm' or unit == '~m':
        return 'um'
    else:
        return unit