Displacer level sensor use Archimedes’ Principle to observe liquid level by unendingly measurement the load of a displacer rod immersed within the process liquid.
The displacer is cylindrical in form with a continuing cross-sectional area and created long or short as needed. Normal heights range from fourteen inches to one hundred twenty inches.
As liquid level can increase, the displacer rod experiences a bigger buoyant force, creating it appear lighter to the sensing instrument, that interprets the loss of weight as a rise in level and transmits a proportional o/p signal.
As liquid level decreases, the buoyant force on the displacer rod decreases with a corresponding weight increase that is understood as decreasing level by the level detector that then provides a corresponding signal output.
Shown below may be a typical displacer level detector installation:
Although the fundamental theory of operation has been outlined higher than, in a very practical displacer level sensor, the construction is built to attain the specified measuring objective with sophisticated electronic circuitry.
In these kinds of displacer level sensors, the displacer is connected to a spring that restricts its movement for every increment of buoyancy (i.e. level change).
A transmitter incorporating a Linear Variable Differential transformer (LVDT) is employed to trace the increase and fall of the displacer rod as liquid level changes. Sophisticated electronics is then accustomed process the voltage signal from the LDVT into a 4-20mA o/p signal.
Archimedes’ Principle Applied to the Displacer
According to Archimedes’ Principle, the buoyant force on AN immersed object is usually equal to the load of the fluid volume displaced by the item.
Suppose in the displacer level sensor, we’ve a cylindrical displacer rod with density, ρ, radius, r, length, L, and method fluid of density, У. during this installation, the length of the displacer rod is proportional to the liquid level being measured:
Volume of displacer rod, V=πr2L
shown within the diagram for the displacer level sensor installation higher than, once the vessel is full, the displacer rod is totally immersed within the method fluid thus the volume of method fluid displaced is V=πr2L. once the level of the vessel is empty or minimum, volume of method fluid displaced is V = zero
Hence:
When container is full, buoyant force on displacer rod is given by:
Buoyant Force = weight of method fluid displaced
= πr2LУg (g = acceleration because of gravity)
Real weight of displacer = πr2Lρg
Net weight of displacer detected by the LVDT, transmitter and associated electronics once vessel is full is:
= πr2Lρg−πr2LУg=πr2Lg(ρ–У)=Vg(ρ–У)
As will be seen higher than, the net weight detected by the LVDT is proportional to the distinction in density of the displacer rod (ρ) which of the method fluid (У)
Therefore, the displacer rod should have the next relative density than that of the liquid level being measured and have to be compelled to be label for the particular gravity of the liquid.
Typical relative density range for liquids wherever the displacer level sensor is applied is within the range of 0.25 to 1.5. Another purpose value mentioning is that the range of the displacer level instrument depends solely upon the length (L) of the displacer rod specified for the given application.
When vessel is empty or level is minimum,
Buoyant force on displacer = zero
Hence, weight detected by the LVDT is =πr2Lρg
The LVDT registers a voltage signal at min. container level and outputs a corresponding signal. The displacer length is decided by the operative range (span) nominative, the specific gravity, pressure, and temperature of the method fluid. The diameter and weight area unit factory calculated to make sure correct operation and providing exact 4-20mA output.
Areas of Application
The displacer level sensor is employed in level measuring applications like knock- out pots, condensation drums, separators, flash vessels, storage vessels and receiver tanks.