External cage switches
A permanent magnet is attached to a pivoted switch actuator. As the float/displacer rises following the liquid level, it raises the attraction sleeve into the field of the magnet, which then snaps against the non-magnetic enclosing tube, actuating the switch. The enclosing tube provides a static pressure boundary between the switch mechanism and the process. On a falling level, the float/displacer deactivates the switch.
Float type caged level switch
All float operated liquid level controls operate on the basic buoyancy principle, which states, "a body (float) immersed in a liquid is buoyed upward by a force equal to the weight of the displaced liquid". As a result, floats follow the liquid surface and move a distance equal to the liquid level movement. Because of this, they are normally used for narrow level differential applications such as high-level alarm or low- level alarm. External cage float type level switch controls use a float and magnetic coupling action that causes switch operation for fluid level measurement.
Displacer type caged level switch
Liquid level movement changes the buoyancy force on a displacer suspended from a range spring. The force change on the spring causes it to extend or compress, moving an attraction sleeve into or out of the field of the switch magnet, actuating the switch.