A thermal conductivity detector system within an incubator having an atmosphere of moisture and CO.sub.2 therein is calibrated this way: in a first phase, the incubator, after ambient air being supplied thereto, is closed with respect to further gas or air admission; in a second phase, and after a predetermined time interval, a CO.sub.2 measurement is made by means of the thermal conductivity cell, and upon deviation of the measured value from a CO.sub.2 zero value, the deviation is stored and utilized for 0.2 correction of the CO.sub.2 measured value. A CO.sub.2 tolerance range of measured values of, for example, .+-.0.2 vol. % CO.sub.2 is determined. After introduction of moisture or humidity into the chamber, a third phase is initiated for a predetermined time interval and run through so often until the CO.sub.2 measured value for the entire time interval remains within the CO.sub.2 tolerance range. After successful termination of the third phase, the moisture of said value of the CO.sub.2 measuring is determined, stored, and utilized for 0.2 correction of the CO.sub.2 measured value. Considering the CO.sub.2 offset, as well as the CO.sub.2 humidity offset value, the 0.2 value of the CO.sub.2 measuring is indicated, and the calibration is terminated, permitting gas valves to be opened--provided a predetermined calibration range has not been exceeded; if it has been exceeded, the calibration has to be repeated, or the range reevaluated. Thereafter, laboratory test samples can be introduced into the interior of the incubator.