Level Measurement

What is a variable displacement level device and how does it measure the level?

The working of this type of level measuring device is based on the Archimedes principle of buoyancy. According to the Archimedes principle, a body that is immersed in a liquid will be buoyed up by a force equal to the weight of the liquid it displaces. There will be an upward pressure which will be around the displacer and this would create a force which is called buoyancy. The control feature of the displacer operated level switch is better than the float operated level switch.

How does a variable displacement device works and how to install it?

A displacer level transmitter is a float or weight connected to a spring balance, when the weight is immersed in a liquid then it would lose weight equal to the liquid weight displaced. By detecting the apparent weight of the float a level instrument can be devised. If the cross-sectional area of the displacer and density of the liquid is constant, then a change in level would make an exact change in displacer weight. The output is then converted into the electrical or pneumatic output of the level.


The displacer can be mounted internally or externally, in the internal mounting it will be suspended in a tank and the external mounting will be done outside of the tank in a float chamber. In the first case, the displacer is directly exposed to the process and indicates the level as buoyancy changes due to the changes in the level of the process liquid. This type of device must be installed externally it can be mounted external to the tank or the vessel so that we can do maintenance without any process interruption. We must provide an air fin extension between the level sensing element and the transmitter for applications where fluid temperature is more than 200 degrees Celsius. The displacer should be installed vertically, the displacer center should be at the elevation at which the level in the tank should be maintained.

How to select a displacer level measuring device?

While selecting the displacer float, we must consider its size and diameter it must be selected according to the size of the tank and the height of the level to be measured. If the liquid level is lower by completely uncovering the displacer then the displacer can’t measure the level. If the level of the liquid is more than the displacer or above the displacer then it can’t measure the level.

How to measure the liquid-liquid interface by a variable displacement device?

In order to do this type of measurement the displacer must be submerged completely, the displacer must be positioned in a way that its mid-section should be at the liquid-liquid interface. So when the interface level changes the buoyant force acting on the displacer will vary the displacer weight. This change in weight is due to the change of the buoyancy which is caused by the difference in specific gravity between the two liquids. The weight of the displacer is then a function of the interface position and also the direct indication of the interface level.

What are the types of variable displacement devices?

Torque tube

In this type, the displacer will be suspended by an arm that would be connected to the torque rod. A bearing would support the torque rod’s movable end, and this type of bearing would provide a frictionless pivot point. The torque tube must be capable to hold the displacer if there is no buoyancy or in case of the minimum level. A hollow tube will transfer the motion of the displacer to an electronic instrument which will make a signal that is proportional to the change in the weight of the displacer.

Spring balance displacer

This device is similar to the torque tube in this the torsional spring of the torque tube is replaced by a conventional range spring. The indicating portion is separated from the process, by locating it in a separate housing. The displacer motion will be transmitted to the indicator by magnetic coupling. The displacer will be suspended to the tank by using an extension range spring. So when the tank level changes then the displacer weight would vary, so the spring would expand or contracts according to the tank level. A magnet would be attached to the displacer rod that would rise or fall according to the movement of the displacer. There is a second magnet which is located in the indicator hosing would follow the displacer magnet and transmit this movement to the electronic unit.

What are the advantages of displacement level measurement?

  • It is good with clean liquids
  • It can be used with a wide variety of fluid density
  • High accuracy
  • It can be placed internally or externally
  • It can be used for liquid interface measurement
  • It is not affected by turbulence or pressure
  • It can be used with viscous or dirty fluids

What are the disadvantages of displacement level measurement?

  • If the tank fluid density varies it would affect the measurement
  • If the specific gravity changes then the weight of the displacer material would change and it would affect the measurement
  • Limited range of measurement
  • These devices are expensive
  • Deposition of the solid in the displacer would affect the measurement
  • It is only used for small tanks
  • It is very difficult to calibrate the displacer

What are the applications of displacement type level measurement?

  • Heavy oils
  • Liquids with solids
  • Agitated fluids
  • It can be used with foaming liquids
  • Chemical plants
  • Paints and varnishes

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