Abstract: 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.

Abstract: The invention relates to a monitoring unit for photovoltaic modules, wherein the modules are designed for a direct conversion of radiation energy, such as e.g. solar energy, into electric energy, and the system is designed function monitored, wherein each photovoltaic module (1-3) has assigned to it its own monitoring unit (4-6; 15, 16; 28) and all monitoring units are centrally monitored and documented by a control computer.