A method and analyzer for determining a measured value of a measured variable of process automation technology in a liquid or gaseous medium by means of an optical sensor includes taking a sample of the medium; mixing the sample with one or more reagents; supplying an emitter of the optical sensor with an exciter signal for producing sent light, wherein the sent light by interaction with the mixed sample is converted into received light as a function of the measured variable; producing a receiver signal by means of a receiver of the optical sensor from the converted, received light; and determining the measured value based on the receiver signal and a calibration function, which includes a term which takes aging of the reagents into consideration.