A confined, fully or partially liquid-filled container having at least one flexible wall, utilizing the effect of gravity on liquid in the container to generate a force that acts on an actuator mechanism to operate a normally-closed control valve. In preferred embodiments, the container is a simple sealed bag partially liquid-filled and air-free.
The flexible-walled container contains a predetermined quantity of liquid. The bag is confined by walls of a housing formed of a non-flexible material, and is affixed to the housing preferably to an upper wall of the housing in any conventional way. The housing is disposed in a vessel for holding a liquid and is having one or more apertures, which are in fluid communication with the interior of the vessel and through which liquid can enter from the vessel into the housing to a level corresponding to a desired level of liquid in the vessel.