They are used wherever it is necessary to control and regulate the level of liquid (e.g. water, oil, etc.). The sensor has a float equipped with a magnet that moves vertically along the guide tube under the influence of the buoyancy of the liquid, causing the reed switch to close or open.

They are usually mounted in the bottom or lid of the tank, so they can be used to control the lower or upper level of the liquid. By changing the position of the float, the function of the reed switch is changed from normally open to normally closed.