What is the difference b/w Float and displacement type level transmitter? Explain the mechanism and How to calibrate them?

 

Quick Answer
Floats Float, Displacers Don't

The float will follow the level so you should be able to calibrate it by simply raising and lowering the float to operate the rheostat, encoder or whatever method it uses to convert travel into output

A displacer is normally too heavy to float, just sits in one place but as the level rises its weight becomes less (Archemedes principle) usually measured by a strain gauge or torque tube. To calibrate this type you need to simulate the change in weight or immerse it in the product.

I'm sure you can google for a manual.

Roy

 

As said, Floats have a density less than that of the fluid in which they are measuring level. Hence a float "floats".

Whereas a displacer has density greater than that of the medium in which it is placed and hence it can't float but "displaces the amount equal to the displacers volume equivalent to the height of the displacer."

Calibration of Float:

1) Isolate the float type LT by closing the isolation valves provided at the legs (up leg and down leg).

2) Drain the LT completely. Now the LT should not have any fluid and the float should be resting at the bottom.

3) Connect the drain with a flexible/non-flexible U-tube. Now when you start filling the flexible leg, the liquid fills the floats containment equal to the level in the U-tube( just like the concept of Level-gauge).

4) Adjust the micro-switch, zero-span screw, potetiometer correspondingly to give you a linear calibration.

Calibration of Displacer type LT:

it works on archemeides principle.

1)Weight of Displacer at 0% level= Weight of Displacer=(pie)*(r^2)*(length of displacer)*(density of displacer)-------------eqn(1)

Weight of liquid displaced= pie)*(r^2)*(length of displacer corresponding to 100% level)*(density of liquid)-------------------------eqn(2)

Weight of Displacer at 100% level= Weight of Displacer-Weight of liquid displaced------------eqn(3)


2) Now remove the LT from flange and bring it to your lab. Take two weights equal to "eqn(1) and eqn(3)"

3) LRV weight = weight in eqn(1)= weight of displacer at 0% level

Apply LRV weight by connecting it to the rod on which displacer hangs. Alternatively, you can use the displacer itself.

4) URV weight = weight in eqn(3)= weight of displacer at 100% level

Apply URV weight by connecting it to the rod on which displacer hangs.

5) Now check your calibration by connecting intermediate weight and calculating the equivalent level that should be shown as would be found in the formula.

Generally for all displacers, the weight of displacer will be mentioned and can be seen when you open the LT and remove the displacer from the LT. It also mentions the radius and Length of displacer.

I hope this solves the problems you are facing during calibration.

Best Regards,
Nikhil

 

Nikhil,

I'm not nit picking but we don't know for sure if the transmitter is in a standpipe, it could be just straight in the tank. I agree the displacer is normally heavier than the liquid but it doesn't have to be. Hopefully the poster can get a manual.

Cheers
Roy

 

Vital and basic: The Float can tell you the level of the liquid in the Tank.

A Displacment device will tell you if there is any liquid in the Tank, or not.