Liquid level control system and method of operating the same

A control apparatus responsive to respective water levels upstream and downstream of a flow restriction located in a water course, the apparatus comprising:

• a bubbler system comprising an upstream gas bubbler and a downstream gas bubbler connected in gas flow communication with each other and to a source of compressed gas, the upstream bubbler having an upstream mouth and the downstream bubbler having a downstream mouth positioned at an elevation below that of the upstream mouth whereby pressurization of the bubblers with both mouths submerged below water provides a common system pressure;
• transducer means connected to the bubbler system and comprising means to generate a control signal in response to the system pressure which is normally determined by the pressure head imposed on the upstream mouth by the upstream water level but becomes determined by the pressure head imposed on the downstream mouth by the downstream water level when, due to lowering of the downstream water level relative to the upstream water level, the magnitude of the difference between the upstream and downstream water levels exceeds the difference in elevation between the upstream mouth and the downstream mouth;
• and
• a water flow regulator located downstream of the flow restriction and responsive to the control signal.

A method for controlling an operating mechanism by means of a control apparatus responsive to respective liquid levels upstream and downstream of a flow restriction in the courseway of the liquid, the method comprising:

• sensing the upstream liquid level and generating an upstream signal which is proportional to the upstream liquid level;
• sensing the downstream liquid level and generating a downstream signal which is proportional to the downstream liquid level;
• generating a control signal in response to the upstream signal unless the magnitude of the difference between the upstream and downstream signals in response to a lowering of the downstream liquid level relative to the upstream liquid level exceeds a pre-selected value, in which case the control signal is generated in response to the downstream signal;
• and
• transmitting the control signal to the operating mechanism to operate the operating mechanism in response to the control signal.