NOVELTY - The sensor (1) has a vibration generator (22) present in an area of a container (20) and for setting a vibration body in an oscillatory movement. A vibration sensor (24) is arranged at a distance from the generator. The vibration body is formed as a separate component (23) in the container. The generator and the vibration sensor are arranged at opposite sides and/or ends of the component and the container. The generator has a rotary mass provided with an unbalanced mass. Discharge of liquid over valve devices (16, 18) is triggered over an electronic unit (19).    USE - Filling level sensor for a container filled with liquid in a water separator i.e. anode water separator, which is utilized for separating product water from exhaust gas of fuel cells of a fuel cell system (all claimed) utilized for providing electrical energy to a transportation unit such as vehicle i.e. motor car, aircraft and ship.    ADVANTAGE - The filling level sensor is simply, reliably, economically and efficiently operated, and provides safe filling level value. The sensor is insusceptible against contaminants. The vibration generator and the vibration sensor are arranged at the opposite sides and/or the ends of the component and the container, thus ensuring simple and direct detection of the filling level of the liquid in the container without falsification of a measured value based on frequencies deviating from each other. The design of the level sensor achieves efficient dual utilization of the container and/or the component as the vibration body, avoids the need of additional installations in the interior of the container and saves designed space and weight of the sensor.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a method for determining a filling level of liquid    (2) a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a water separator with a filling level sensor.    Filling level sensor (1)    Valve devices (16, 18)    Electronic unit (19)    Container (20)    Vibration generator (22)    Separate component (23)    Vibration sensor (24)