Abstract: In a method for determining a measured variable of a measured medium, wherein the measured medium is brought in contact with an indicator or an indicator mixture, whose absorption spectrum has a first, and a second, wavelength range, which essentially do not overlap, a first light source (6) is activated for emitting a first light signal with a wavelength from the first wavelength range and a second light source (7) is activated for emitting a second light signal with a wavelength from the second wavelength range, wherein the intensity of the first light signal is modulated by a first and the intensity of the second light signal by a second, periodic signal, wherein at least a part of the first light signal and at least a part of the second light signal propagate as first and second measurement light signals along a measuring path and are transformed on the measuring path by optical interaction with the indicator or the indicator mixture to transformed measurement light signals, and wherein a total intensi

Abstract: A sensor for registering a measured variable of a medium, especially in a bioprocess. The sensor includes a sensor body, wherein at least one surface section of the sensor body can be supplied with the medium, and wherein a condition of this surface section affects the measured value. The sensor is characterized by the fact that the surface section contains a substance with biocidal properties.