A water level indicator is a measuring or monitoring device used to detect and indicate the height or quantity of water inside a storage vessel, such as a water tank.
 
Function of a water level indicator
 
Its main function is to show how much water is currently contained in the tank and, depending on the type, may provide a visual, electrical, or digital indication.
 
Typical functions include:
 
Detect water level — Determines the level of the water inside the vessel.
Monitor filling — Helps the operator know when the tank is approaching its maximum capacity.
Monitor water usage — Shows when the water level is becoming low and refilling is required.