Wet Leg Pressure Level Measurement


Thread Starter


We have a review for an upcoming exam, and in my studies I can't seem to come to a conclusion of the answer. The question asks,

"For a wet leg pressure level measurement system, if the wet leg seal fluid starts to evaporate, will the level signal change? If yes, will it increase or decrease? Briefly explain."

As the seal fluid should have a greater S.G than the process fluid, in this case upon the evaporation of the seal fluid, the process fluid will cause in an increase in the level transmitter reading?

Any help would be greatly appreciated,

Thank you!

Bruce Durdle

(There is no reason why the seal liquid should have an SG greater than the process fluid - if the process fluid is sulphuric acid (SG = 1800) it's most unlikely.)

Break the problem down into small steps.
The differential pressure is proportional to the difference in height between the reference column in the wet leg and the liquid in the tank.

If the reference leg liquid evaporates, the level of liquid in the reference leg will fall. So for a given process liquid level, the differential pressure developed will fall.

If the reference leg liquid was at the correct point, this drop in differential pressure would be developed only if the level of liquid in the process leg rises.


the best way to understand what is happening to the signal is to make a sketch of the tank/transmitter and calculate the pressures the transmitter sees on each input for the various conditions . P = height * density * G dP = Ptank - Pwetleg.