Disclosed is an apparatus for treating containers with a transport device which transports the containers to be treated along a predetermined transport path, wherein said transport device has a rotatable transport carrier at which a plurality of treatment stations for treating the containers is arranged, wherein the apparatus has a clean room within which the containers are treated in a predetermined manner, and with a sealing device which seals said clean room from an environment, wherein said sealing device has a reservoir which can be filled with a liquid medium, and with a sensor device for measuring a filling level of said liquid medium at least indirectly. The sensor device is arranged in a supply line which can be brought into flow connection with the reservoir.