Displacer switch technology provides accurate level measurement across all process industries today. To determine if this is the right solution for your facility, it is important to learn about displacement type liquid level measurement – and how this technology functions within industrial applications.
How a Displacer Switch Works
Displacement type liquid level measurement is based upon a basic principle – that the buoyancy of an object equals the force of gravity on the liquid that is displaced by the object. As a result, the buoyancy force reduces the weight of the displacer.
A range spring supports the weight of each displacer. When you submerge a displacer, the spring responds to the weight change by moving upward in a vertical direction.
A range spring will move when the liquid level rises on a displacer. However, the range spring movement (see A in the illustration) is only a small fraction of the level traveled between displacers (see B in the illustration).
The displacer’s range spring is also connected to a magnetic attraction sleeve and operates inside of a non-magnetic enclosing tube. When the range spring moves, it forces the magnetic attraction sleeve to enter the magnetic field of the pivoted switch mechanism’s magnet, which makes or breaks the switch mechanism’s contacts.
When a liquid’s level falls below the displacer, the buoyant force discontinues and the range spring extends downward. This also causes the magnetic attraction sleeve to move downward in a vertical direction – and out of the magnetic field of the switch mechanism – which reverses the process.
It is also important to note that displacer switches contain a built-in limit stop. This feature prevents excessive stroking of the range spring when levels surge within the application.
Benefits and Limitations
There are many advantages of liquid level measurement using mechanical buoyancy technology, including:
- Provides a proven, highly reliable solution.
- Does not require electrical power – or calibration.
- Enables easy setup without level change.
- Delivers multiple stage level control.
- Offers narrow and wide switch options – as well as transmitters for continuous output.
- Handles service pressures up to 5000 psig (345 bar) and service temperatures to +1200° F (+649° C).
Certain process conditions may interfere with the mechanical buoyancy technology. These conditions include solids, moving parts and dirt or buildup. In addition, switch accuracy is limited to ±0.25 inches (6 mm).
Follow Our Displacer Switch Discussion
Next week, we will examine how displacer switches can provide an effective displacement type liquid level control solution for sump applications in numerous industries. For more information about displacer switches, including the Magnetrol® Series A10, Series A15 (single stage) and B15 (dual stage), you can also download the MAGNETROL Buoyancy Products technology bulletin.