Level differential controller principle?

Level differential control is sometimes used to regulate flow in open channels. The relationship of liquid height through a venturi flume to liquid flow is well-established. Differential level can then be used to change a valve position or a pump speed in the flow route in order to keep flow constant. The meaurement technique is often ultrasonic, though not always.

The term "level control valve" needs expanding before any comparison can be made. What is the measurement technique involved in operating this valve?

The humble ballcock Portsmouth Valve in a domestic water header tank might be referred to as a "level control valve", for example.