Principle
A float rises and falls with the liquid level, traveling up and down a float rod between two collars. If the liquid rises above the level fo the upper collar, or falls bellow that of the lower collar, the movable float rod is moved upwards or downwards respectively by the buoyancy or weight of the float. This trips a microcontact. This contact can be used for the function of "filling" or "emptying" according to the choice of connection terminals.
For example, in the case of "emptying" the rod pushes upwards when the upper collar is reached. This trips the microcontact, wich can, for instance, switch a pump. Liquid is pummped off. The float falls with the liquid level and pulls the float rod down again when it reaches the lower collar. This breaks the contact an the pump is switched off.
In the "filling" mode, the arrangement is the other way round.

This units is not suitable for use in turbulent or corrosive liquids.