Wet chemical cleaning of silicon wafers creates unwanted nitrous gases. To make sure these gases do not exceed statutory limits, the exhaust flow is treated in a downstream exhaust air cleaning system. Flue gas scrubbers are one of the key steps in the treatment process. Washing fluid is pumped in a circuit via a pump station and sprayed finely into the scrubber to bind air pollutants to the liquid. The pollutant-laden washing fluid is periodically replaced with fresh water. In the storage tank, the level of the washing fluid is continuously monitored and kept constant during the process. The level measurement must be independent of the mixing ratio of the washing fluid as well as vapours and condensation in the tank and provide reliable measurement data.