It is a type of buoyancy operated method Uses the theory of Archimedes principle, which states that “The force produced, when a body is submerged into liquid with a constant density is equal to the fluid displaced”. That means, when a body is fully or partially immersed in any liquid, it is reduced in weight by an amount equal to the weight of the volume of liquid displaced.
According to Archimedes principle, the displacer, when submerged in liquid, will ‘lose’ its weight, and this weight loss is proportional to the level of the liquid. Thus, a level, density, or interface level change in the measured fluid causes a change in the displacer position. This change is transferred to the torque tube assembly. As the measured fluid changes, the torque tube assembly rotates and the indicating needle attached to the torque tube will have indication. movement then transferred through c type burden tube which has a little hole to create back pressure move flapper nozzle arrangement. Then that signal transferred and amplify for the measurement.
To understand in better way kindly have a look at attached video. You can clearly see how things work in this detailed explanation.
This rotary motion can then be extend for remote signal and indication. It is transferred to the transmitter level assembly. The rotary motion moves a magnet attached to the lever assembly, changing the magnetic field that is sensed by the position sensor. The sensor then converts the magnetic field signal to an electric signal. The current drier circuit in the transmitter develops a 4~ 20 mA signal proportional to the dc amplifier voltage output.
AREAS OF APPLICATION
The displacer level sensor is used in level measurement applications such as knock- out pots, condensate drums, separators, flash vessels, storage vessels and receiver tanks.