The method and analyzer for the determination of a measured value of a measured variable of process automation technology

The invention relates to a method for determining a measurement value of a measured variable of process automation technology in a liquid or gaseous medium by means of an optical sensor, the at least one transmitter (17.1), for transmitting light having at least two wavelengths (1-7), and (a transmitter having 17.1) associated receiver (17.2) for receiving received light, comprising the steps of: taking a sample (13) of the medium (15); Mixing of the sample (13) with one or more reagents (16); Applying the transmitter (17.1) with an excitation signal for the generation of the transmitted light, the transmitted light by the interaction, in particular by absorption, with the mixed sample (13) as a function of the measured variable is converted to the received light; Generating a receiver signal by means of the receiver (17.2) from the converted received light; Determining the measurement value from the receiver signal and a calibration function. The method is characterized in that the aging of the reagent is taken into account in determining the measurement value, in particular the calibration function comprises a term of the aging of the reagents considered. The invention further relates to an analyzer.