A known metering system for automatically maintaining a predefinable concentration of a liquid corrosion inhibitor in the aqueous hydraulic liquid of a water-hydraulic hydraulic system is characterized in that the measuring circuit can be disconnected from the hydraulic system (controllable valves 14, 15), in that the measuring circuit can be short-circuited between the outlet line and the inlet line by means of controllable valves 16, 17 and can be connected to a source (cleaning tank 18) of a cleaning liquid for a cleaning circuit. As a result, a cleaning liquid for cleaning the concentration sensor can be circulated in the measuring circuit for a predefinable time. For the purpose of calibration, the metering system can be charged with a reference liquid with a known inhibitor concentration, in particular water, in particular water from the public supply. Further sensors (23-25) can be connected into the measuring circuit.
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