The level of the liquid present in a vessel is checked by one or more displacers (i.e. floats correctly ballasted) hung on a metallic rope.  When level rises up to the preset height, the displacer makes the output device trip (trip on rise); when level comes down again and exceeds the preset point, the displacer makes the output device come back to the initial position (rest on fall); between the trip and reset points there is always a gap, named differential : see below.  The inverse function is available too: Trip on fall and Reset on rise. The output device can be electric or pneumatic, is snap action and is placed in the housing.   All of them are mounted on top vessel.

Several types of Switches are available, with some elements in common:

• Body and flange in the ratings ANSI 150, 300, 600 psi, in carbon steel ASTM A106B (body) and ASTM 105N (flange), stainless AISI 304, or stainless AISI 316.  Flange : ANSI or UNI/DIN standards .
• Displacer and rope (L=1÷10m) in stainless AISI 316.  The displacer can be fixed on the rope at the desired height, decided by the user himself. 
• For liquids with specific gravity from 500kg/m³ up.
• Housing and housing-holder : as described below.
• Outer finish : green, for marine and tropical climates.