Model 740A Float Switch

The Delta Controls Model 740A provides alarm switch action at two points as a liquid level varies. 

The unit is located on the side of the vessel with the float extending into the vessel. The float rides on the surface of the liquid and is carried up and down as the liquid level varies.

The attractor is coupled to the float by the linkage and also moves up and down as the level varies. The attractor is located inside of the barrier tube and contacts the process fluid. 

The switching stations are located on the outside of the barrier tube and are isolated from the process fluid.

Each switch station contains a magnet whose magnetic lines of force penetrate the nonmagnetic barrier, and are magnetically attracted to magnetic materials such as the attractor. 

The magnet is pulled in against the tube as the attractor moves up into its magnetic field. The magnet is connected to an output switch which actuates as the magnet is pulled in against the barrier tube.

The bottom switch station actuates as the float begins to move downwards from its highest position.

Continued downwards movement lifts the attractor up into the field of the top switch station magnet and that station actuates.

Upwards movement of the element pulls the attractor out of the top station's, and finally the bottom station's magnet field, and they deactuate.