[en] At least two calibration inserts are placed in the detection unit body, thus forming a secondary standard which is calibrated using the calibration curve of the given detection unit determined once for all by means of ready made primary standards. The calibrated secondary standard can be used for repeated determinations or for calibration curve tests whenever the calibration unit is used in the field irrespective of time intervals between the individual measurements. Thus, a costly repeated determination is avoided of the calibration curve at a special laboratory using primary standards. (J.P.)