Patent ID: 7268352

Claim:
An apparatus for infrared-optical determination of the concentration of a substance in a gas sample, having the following characteristics: a modulated radiation source; a thermal detector; a measurement chamber between the radiation source and the detector, as a measurement distance for recording the gas sample; means for splitting the measurement signal, furnished by the detector, into a direct voltage component and an alternating voltage component; means for forming averaged transmissions T DC 900 , T AC 900 of the direct voltage component and of the alternating voltage component; calibration curves, for the functional relationship between transmission and concentration, associated individually with the transmissions of the direct voltage component and the transmissions of the alternating voltage component; and means for forming a correction value for the direct voltage component, by forming the correction value ΔT from the deviation of a comparison variable T DC , korr 900 , formed from the averaged alternating voltage component T AC1 900 with the calibration curves, from the associated, averaged direct voltage component T DC1 900 .