Gems Sensors & Controls LS-750 Operating Manual
Model LS-750
Steel Tank Sensor
Instruction Bulletin No. 157728
Environmental Products
These Sensors may not be compatible with indicating
and alarm equipment supplied by other manufacturers
Note: LS-750 sensors are non-voltage producing devices and do not contain energy storing
components. However, since primary use is in hazardous locations, an appropriate intrinsically safe interface device is required.
With its compact size, the Gems LS-750 single float, liquid level sensor is ideally suited for use in steel doublewall tanks. It requires no calibration, and is easy to install and maintain. When positioned vertically at the bottom
of a steel tank’s stand pipe, it reliably senses the presence of a liquid. It detects hydrocarbons and water as low
as 3/4" from the bottom of a tank or sump. The LS-750 sensor features an epoxy-encapsulated design providing
an environmental seal, that makes it a fine choice for harsh environments. An integral slosh shield guards the
float from debris; thereby assuring dependable service.
Sensor Operating Principle
Gems LS-750 liquid level sensor operates on a direct, simple
principle. A float is equipped with powerful, permanent magnets. As the float rises or lowers with liquid level, it actuates a
magnetic reed switch mounted within the stem. This condition either opens or closes the electrical circuit to operate an
external alarm or control circuit. When mounted vertically,
this basic design provides a consistent accuracy
of ±1/8th inch.
N
Permanent
Magnet
Float
Hermetically
Sealed Magnetic
Reed Switch
Reed
Switch
N
Envelope
S
S
Glass
S
MAGNET
N
Dimensions
3-1/2" Ref.
(88.9 mm)
Retaining Ring
Cable
!
1-7/16" Ref.
(36.5 mm)
Sealing
Nut
Liquid-Tight
Fitting
Slosh
Shield
3/4" Actuation
(19.0 mm)
Float
!
!
!
Note: Please refer to specific Gems outline drawings for
operational specifications.
Read all instructions before beginning - Follow all safety precautions
Barricade the area
Do not allow vehicles
or unauthorized people
in the work area
Do not smoke or allow
open flames in the
work area
WARNINGS
Read the instructions and warnings carefully before installing the sensor. This unit must be installed in accordance with National Electrical Code ANSI/NPFA-70, 1990; as well as Federal, State and local codes and any
other applicable safety codes.
1. To avoid electrical shock, which could kill you, be sure AC power to monitor is off during installation.
2. The nature of the sensor is that it is a non-voltage producing device, containing limited energy-storing
components. However, since its primary use is in a hazardous location, an appropriate intrinsically safe
interface device must be used.
Dangerous environment.
Failure to install this equipment in
accordance with NFPA 30A and
NFPA 70 could result in severe
injury or death.
Read, understand and follow
NFPA 30A and NFPA 70.
Note: Failure to observe these warnings could result in serious injury and
death, as well as undetected potential environmental and health hazards.
Typical Wiring Diagrams
Non-Isolated System – Single Zener Barrier
Hazardous
Area
Black
Red
N.O. Dry
or
N.C. Dry
2-Conductor
Cable
If two signal lines must be maintained at above ground potential, an individual zener barrier is required per single line.
Single Safe-Pak® Relay
Non-Hazard-
ous Area
3
21
Fuse
( )
(+)
DC
Power
Supply
Annunc.
N.O. Dry
N.C. Dry
Hazardous
Area
Safe-Pak® is an intrinsically
safe, solid-state relay.
N.O. Dry
or
N.C. Dry
Black
Red
2-Conductor
Cable
Isolated System – Dual Zener Barrier
Hazardous
Area
Black
Red
or
2-Conductor
Cable
Non-Hazard-
ous Area
Load
SAFE-PAK
Non-Hazard-
ous Area
Supply Barrier
3
2
3
2
Fuse
Fuse
1
1
Fuse
Signal
Return
Barrier
Hot VAC
Neutral VAC
Sense
Resistor
( )
Power
Supply
Annunc.
(+)
DC
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