Convert detector serialnumber to string representation (OID)

def get_det_oid(self, det_id):
        """Convert detector serialnumber to string representation (OID)"""
        try:
            return self.detectors[self.detectors.SERIALNUMBER == det_id
                                  ].OID.iloc[0]
        except IndexError:
            log.critical("No OID found for det ID '{}'".format(det_id))
            return None