4
46-620 B73 & Series 75 Liquid Level Switches
2.0
Reference Information
2.1
Description
Magnetrol’s B73 and Series 75 level switches are float oper-
ated units suitable for use on clean liquid applications for
level alarm, pump control and safety shutdown functions.
Series 75 units are available with tandem floats for applica-
tions where widely spaced high and low switching are
required by a single control.
2.2
Theory of Operation
The design of float operated level switches is based upon
the principle that a magnetic field will not be affected by
non-magnetic materials such as 316 stainless steel. In this
case, the float moves a magnetic attraction sleeve within
a non-magnetic enclosing tube and actuates a magnetic
switch mechanism. The enclosing tube provides a pressure
seal to the chamber and therefore to the process.
2.3
Operating Cycle
As the liquid level rises in the chamber the float moves the
magnetic attraction sleeve up within the enclosing tube
and into the field of the switch mechanism magnet.
Refer to
Figure 4
. As a result, the magnet is drawn in
tightly to the enclosing tube causing the switch to trip,
making or breaking an electrical circuit. As the liquid level
falls, the float drops and moves the attraction sleeve out of
the magnetic field, releasing the switch at a predetermined
low level. Refer to
Figure 5
. The tension spring ensures the
return of the switch in a snap action.
Tandem float units incorporate two floats which operate
independently. The lower float actuates the upper switch
mechanism, and the upper float actuates the lower switch
mechanism. The upper float is attached to the lower
attraction sleeve by means of a hollow stem. The lower
float attaches to the upper attraction sleeve with a solid
stem, which extends upward through the upper float and
stem assembly.
4
5
6
Swing in
position
Mercury switch
Return spring
Enclosing tube
(non-magnetic)
Attraction
sleeve
Magnet
HL
Figure 4
Switch Tripped
Figure 5
Switch Released
4
5
6
Swing out
position
LL
