B73 External Cage
Liquid Level Switches
®
INSTRUCTION MANUAL AND PARTS LIST
DESCRIPTION
B73 instruments are float operated liquid level controls
designed for external float cage mounting.
These instruments are ideally used for off-on liquid level
control, alarm and safety shut-down service in a wide vari-
ety of applications including large steam-electric generating
stations, oil refineries and chemical processing operations.
The B73 is available with either a carbon steel or stainless
steel float cage and is suitable for service pressure ratings
up to 20.7 bar (300 PSIG) and specific gravity ratings down
to 0.60.
OPERATING PRINCIPLE
A permanent magnet is attached to a pivoted switch mech-
anism. As the float rises following the liquid level, it raises
the magnet attractor into the field of the magnet, which in
turn snaps against the non-magnetic barrier tube, tilting the
switch mechanism. The barrier tube provides a static seal
between the switch mechanism and the float.
4
5
6
4
5
6
Switch
mechanism
Magnet
Attraction
sleeve
Return spring
Swing in
position
Swing out
position
HL
LL
Enclosing tube
(non-magnetic)
OPERATING CYCLE
As the liquid level rises in the chamber, refer to Figure 1,
the float moves the magnetic attraction sleeve up within the
enclosing tube and into the field of the switch mechanism
magnet. As a result, the magnet is drawn in tightly to the
enclosing tube causing the switch to trip, making or break-
ing 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 2. The tension spring ensures the return of
the switch in a snap action.
Figure 1
Switch Tripped
Figure 2
Switch Released
Agency
Approval
➀
CENELEC
EEx d
II
C T6, explosion proof
EEx ia
II
C T6, intrinsically safe
➁
BASEEFA
Ex d
II
C T6
CSA
Non-Hazardous CSA Type 4X
➀
Class
I
, Div. 2, Groups B, C & D
Class
I
, Div. 1, Groups C & D
Class
II
, Div. 1, Groups E, F & G
Class
I
, Div. 1, Groups B, C & D
Class
II
, Div. 1, Groups E, F & G
FM
Non-Hazardous NEMA 4X
➀
Class
I
, Div. 1, Groups C & D
Class
II
, Div. 1, Groups E, F & G
Class
I
, Div. 1, Groups B, C & D,
Class
II
, Div. 1, Groups E, F & G
SAA
➀
Ex d
II
C T6 (IP65)
AGENCY APPROVALS
➀
Not available with all switches; Consult factory for proper
model numbers.
➁
Consult factory for proper model numbers.
