Jerguson® External Cage Displacer Operated Level Switch

Jerguson® products are also available under the Magne-Sonics® brand as required based on approved vendor specifications.

top mount displacer operated level switch


• Tri-Magnet Switching
• Vibration Resistant
• Weatherproof
• Flameproof
• Sealed (shown) or Flanged Cage Body
• 316 SS Trim
• ASME B31.1/B31.3 Design


The displacer element is made of either 316 Stainless Steel or Ceramic, is suspended on a Stainless Steel Rod from a spring.  The displacer element is always heavier than its equivalent volume of the liquid in which it is to operate, so it will extend the tension sprint at all times.  In free air, the spring will be extended to a known length, controlled by a mechanical stop to prevent overstressing.  Attached to the spring is a rod and magnet assembly, free to move up and down as the spring extends or contracts within the pressure tube.

A switch mechanism is mounted inside the enclosure adjacent to the pressure tube.

As liquid rises to cover the displacer element, a buoyancy force is created equal to the weight of the liquid displaced.  This force in effect is seen by the spring as a reduction in weight, causing the spring to contract, hence moving the magnet upwards inside the pressure tube and actuating the switch mechanism.  On a falling liquid level, the displacer element is uncovered and the spring sees an increasing effective weight, causing the spring to extend and move the magnet to re-set the switch mechanism.

Switching is achieved with the unique “tri-magnet” system, giving the snap-action “latch-on” switching.


The following video depicts the benefits of the Tri-Magnet level switch system compared to a standard micro-switch or mercury-switch mechanism in external cage level switches:



Brochure: External Cage Displacer Operated Magnetic Level Switch Brochure

IOM: External Cage Displacer Installation, Operation and Maintenance Manual