The displacer “cage” is the vertical pipe immediately behind and below the head unit. The instrument itself appears on the right-hand side of the photo, topped by a grey-colored “head” with two pneumatic pressure gauges visible. The following photograph shows a Fisher “Level-Trol” model pneumatic transmitter measuring condensate level in a knockout drum for natural gas service. The only factor that matters is the density of the process fluid, since buoyant force is directly proportional to fluid density (\(F = \gamma V\)). It should be noted that static pressure inside the vessel will have negligible effect on a displacer instrument’s accuracy. The displacer’s apparent weight reaches a minimum when it is fully submerged, when the process liquid has reached the 100% point inside the cage. The weight-sensing mechanism detects this buoyant force when it perceives the displacer becoming lighter, interpreting the decreased (apparent) weight as an increase in liquid level. Remember that the displacer is too heavy to float, so it does not “bob” on the surface of the liquid nor does it rise the same amount as the liquid’s level – rather, it hangs in place inside the cage, becoming “lighter” as the buoyant force increases. This will submerge more of the displacer’s volume, causing a buoyant force to be exerted upward on the displacer. If liquid level inside the process vessel rises, the liquid level inside the cage rises to match. These two pipe connections ensure the liquid level inside the cage matches the liquid level inside the process vessel, much like a sightglass.
It hangs within a pipe called a “cage” connected to the process vessel through two block valves and nozzles. The displacer itself is usually a sealed metal tube, weighted sufficiently so it cannot float in the process liquid. Process piping in and out of the vessel has been omitted for simplicity – only the vessel and its displacer level instrument are shown: In practice a displacer level instrument usually takes the following form. As liquid level increases, the displacer experiences a greater buoyant force, making it appear lighter to the sensing instrument, which interprets the loss of weight as an increase in level and transmits a proportional output signal. Displacer level instruments exploit Archimedes’ Principle to detect liquid level by continuously measuring the weight of an object (called the displacer) immersed in the process liquid.