Add an additional DATA_VALUE column with converted VALUEs

def _add_converted_units(self, dataframe, parameter, key='VALUE'):
        """Add an additional DATA_VALUE column with converted VALUEs"""
        convert_unit = self.parameters.get_converter(parameter)
        try:
            log.debug("Adding unit converted DATA_VALUE to the data")
            dataframe[key] = dataframe['DATA_VALUE'].apply(convert_unit)
        except KeyError:
            log.warning("Missing 'VALUE': no unit conversion.")
        else:
            dataframe.unit = self.parameters.unit(parameter)