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Interface transmitter displacer type
interface measuring

hi every one

i have an masoneilan level transmitter .
i want to measuring interface of water - oil .\
i have read in manual that displacer must always be immersed in lower density fluid.

why? and how can i be sure about displacer being immersed.

thank u

Since a displacer transmitter actually detects the change in buoyancy force acting on the displacer, when detecting the interface of two liquids the displacer transmitter must be designed and calibrated to ignore the buoyancy force of the upper liquid and detect only the change in buoyancy force caused by level change of the lower liquid.

Buoyancy force is the mass of the displaced liquid. When the displacer is completely immersed in the upper liquid, the volume of liquid displaced, and therefore the corresponding buoyancy force, is a known value. If part of the displacer is above the liquid level, the buoyancy force is a lesser variable value. This variation will cause error in the interface level reading.

The only way to ensure that the displacer is always completely submerged is to use it in a situation where the thickness of the upper liquid layer is always greater than the length of the displacer.

Other technologies, such as guided wave radar, are not limited by the thickness of the upper liquid layer.

i really appreciate your consideration.

but i have seen some data sheets of customers that mentioned that they need level transmitter displacer type for their interface application. they have even mentioned about their fluids.

your tip that said thickness of upper fluid should have been larger than the displacer length is true. but this means that i have another range length at the top of my device to support the volume of upper fluid?

When you are measuring interface level with displacer, 1st consideration is make sure that 100% of the displacer range (i.e C-C distance) is covered with lower density upper fluid. At this point you adjust the displacer transmitter and make transmitter output as 4mA corresponding to 0% of the interface level.

After this cover the 100% of the displacer range by heavier fluid and set the transmitter output to 20 mA corresponding to 100% of the interface level.

That's all. After this when the interface level varies from 0% to 100%, you get correct reading of the interface level. The key is the zero adjustment when the interface level at 0%. If the displacer is not fully submerged in upper fluid then the force on the displacer will vary (depending on the height of the upper fluid) and the zero compensation done will change resulting in inaccurate reading.
This is true even when measuring interface level with DP transmitter. In this case the upper tap shall always be in the lower density of fluid similar to displacer measurement.

The theory of displacer is when a body is immersed in a fluid, it loses weight equal to the liquid weight displaced (Archimedes Principle).

By detection of the apparent weight of the immersed displacer, a level instrument can be devised. If the cross sectional area of the displacer and the density of the liquid is constant, then a unit change in level will result in a reproducible unit change in displacer weight.

And it has disadvantage when its density changed
Since the displacement of the body (it's weight loss) is equal to the weight of the fluid displaced. If the specific gravity changes, then the weight of the displaced material changes, thus changing the calibration.

This is especially problematic in interface measurements, where both liquids increase or decrease density, while the signal is proportional to the density difference.

finally, you have to calibrate again for make sure your measurement.