External Liquid Level Switch
Model S

The Model S is an external service, side mounted liquid level switch. Models are available for service up to 3000 PSIG, operating temperature up to 1000°F, and specific gravity as low as 0.375. The many optional and special constructional features make the Type S practical for a wide range of applications.


Float Chamber: The welded chamber is available in carbon steel, 304 SS, or 316 SS.
Float: Float material is standard in 300 series stainless steel.
Trim: The trim, which includes the float rod with its related hardware, is standard in 300 series Stainless Steel. The enclosing tube assembly is standard in 316 Stainless Steel.
Process Connections and Mounting: The standard connections are 1" located on the side and bottom having an 11-1/2" centerline dimension. Male connections usually ease field installation by eliminating the need for an extra union. Other connections are available as follows: female, socket weld, or flange. Specify in the RFQ with your required centerline dimension.
Differential: With a single switch, the standard differential is approximately 5/8". Wide differential options are available. Field adjustment of the actuating points can be made by raising or lowering the switch on the enclosing tube.
Switch Housing: A water-tight/dust-tight or hazardous rated housing is available. Hazardous housing is rated to:

  • Class I, Groups B,C,D
  • Class II, Groups E,F,G
  • Class III
  • Class I, Zone 1, AEx d IIC
  • Ex d IIC
  • Type 4X
  • IP 66
  • 4X, 7BCD, 9EFG

and listed as:

  • FM Standard 3615
  • UL Standard 1203
  • CSA Standard C22.2 No. 30
  • NEMA Compliance
  • UL 60079-0/UL 60079-1
  • CSA 60079-0/CSA 60079-1
  • ATEX Directive 94/9/EC
  • UL 50
  • IEC 60529

Conduit Connection: 3/4" NPT is standard, 1/2" NPT or M20 are available.

Switches: See Model OF or Z.

Optional Features:

Cooling Fins: For application with a process temperature above 400°F, cooling fins are used to dissipate as much as 200°F which would otherwise be transmitted to the switches.
Wide Differential: For applications with a differential greater than provided by the standard model, the following options are available:

  1. Two Switch: With two switches up to 4 1/2" of differential can be obtained.
  2. Sliding Piston: With this design, the piston travels along the rod, making contact with high and low-level stops. The magnet is strong enough to hold the piston in place while the liquid level raises and lowers the rod. The maximum differential with this design is three inches.
  3. Sliding Float: With this design, the float travels along the rod, making contact with high and low-level stops. This design is used for differentials larger than 4".

Interface: The controls are available for interface service. The difference in specific gravities between the two liquids must be 0.20 or more. The float for interface service is weighted in order to sink in the lighter liquid, but float in the heavier liquid. If the difference is between 0.12 and 0.19, a custom design is available.
External Displacer: For low level applications where the operating pressure is 1000 PSIG or more, an external displacer control can be used. The operation of a displacer level switch differs from a float control, because the operation is based on weight displacement rather than buoyancy. In the low level position, the weight of the displacers will expand the displacer spring and keep the actuating piston out of the magnetic zone of the switch assembly. As the liquid rises, it begins to displace the weight of the displacers. The spring senses the weight change and will compress, lifting the actuating piston into the magnetic operating zone of the switch assembly. In doing so, a plunger is depressed, actuating a micro switch. As the level descends, the switch assembly will stay actuated until the weight of the displacers is sufficient enough to stretch the spring and pull the piston out of the operating zone. This will release the permanent magnet, re-setting the switch assembly to its unactuated position.


Starting in the unactuated position, a rising liquid level will lift the float and rod, raising the actuating piston towards the magnetic operating zone of the switch assembly. A permanent magnet, located within the switch assembly, snaps forward from the magnetic attraction. In doing so, a plunger is depressed, actuating a micro switch. When the level decreases, the float falls away, pulling the actuating piston to its original position, releasing the permanent magnet, re-setting the switch assembly to its unactuated position.