They work on the resistivity of liquids and sense the level via the electrodes placed in the conductive liquids.
The presence of liquid between the electrode (or electrodes) and the metal surface of the container (or earth electrode) causes the functioning of the electronic circuit in the control unit and the subsequent commutation of the output relay. They are used as level controls in tanks, heaters deep wells and other containers and as alarm units for overfilling and the checking of ice accumulation.