The system work like this..
* Set a setpoint for the water level (eg. 200mm).
* Water is pump into the tank.
* The water flow into the tank controlled by flow control (Valtek Flowserve - 3400IQ Digital Positioner).
* The differential pressure sensor checking the water level.
* The flow adjusted to have the same level which set earlier.
* The flow will maintained when it achieved the level needed.