| A level control valve controls the level of a tank by means of either releasing fluid from the tank or adding fluid to the tank depending on whether it is on the inlet or the outlet. The simplest example is the float mechanism in a toilet tank.
Although the float mechanism can be adapted to larger industrial applications, usually, some kind of level measurement device detects the level and sends that information to the control system which then signals the level control valve to open or close. These level measurement devices can be as simple as a pressure sensor on the bottom of the tank. Alternate methods include radar level measurement or nuclear level gauges based on the absorbtion of alpha and beta particles. | 
