Patent ID: 8830470

Claim:
A method for measuring a concentration of at least one gas component in a measurement gas, comprising: guiding light from a light source along a light path through a measurement volume containing the measurement gas to a detector unit; determining the concentration of the gas component from a detected gas component-specific wavelength-dependent absorption of the light; guiding a region of the light path outside the measurement volume through a closed volume which holds a substitute gas without constant gas supply and discharge, the substitute gas being used with a substitute gas component in a predetermined concentration, the substitute gas component being one of not present in an atmosphere and the measurement gas, present in the atmosphere and the measurement gas in a concentration of at least half the predetermined concentration, and present in the measurement gas at a known ratio to the gas component to be measured; monitoring the concentration of the substitute gas component using a detected substitute gas component-specific wavelength-dependent absorption of the light; and generating an error message in an event of a concentration decrease of the substitute gas component exceeding a predetermined amount.