def get_values_missing(meta_data, sep=', '):
    """
    Read the meta data and for each column return the values used to indicate missing data.
    :params meta_data: data frame with the meta data of values
    :params sep: The separator of values
    :return: a dictionary where the keys are column names and values are a list
    """
    x = meta_data.loc[meta_data.Meaning == 'unknown', ['Attribute', 'Value']]
    x.loc[:, 'Value'] = x.loc[:, 'Value'].str.split(sep)
    na_dict = x.set_index(['Attribute']).loc[:, 'Value'].to_dict()
    return na_dict