There are many variables that need to be considered.

They include:

·        Ambient temperature range

·        Temperature range of liquid in the tank

·        Vapor pressure and/or temperature at which the liquid will condense and freeze in the low pressure leg

If you want to keep the leg full, connect a 1 gph purge meter to the bottom of the low pressure leg.
If the liquid in the tank has water in it, water can be used as the purge liquid; however, if the tank is in a location where the ambient temperature goes below 0 degrees C, the leg must be traced and insulated. If the tracing if too extensive, the purge water will begin to boil and the level transmitter will give a false reading.
If the tank has an organic liquid in it that could be used as the purge liquid provided, it doesn't freeze or vaporize too quickly.
Better yet, move the tank to a warm climate like SW Florida. Nothing freezes here. Besides, the way the economy is going, I could use a part time job.

A better or easier method might be to keep the low level leg empty.
If the liquid vaporizes at a relatively low temperature, steam tracing the line works.

Or, purge the line with Nitrogen or some other inert gas.
Another trick is to connect the low side of the d/p to a pipe well above the tank; so that no liquid can get into the leg.

If none of these methods work, put chemical seals on both sides of the d/p transmitter.

Or, don't use a d/p transmitter.

There are capacitance probes, nuclear, sonar,.........................

In the old days, we got a guy with a long stick to put it and his head in the tank. Then he could phone the reading back to the control room. If the operator in the control room didn't like the answer, he would tell the field operator to get a different reading.  This tried and true method works as long as too many operators don't fall off the tank ladder or get poisoned from the tank fumes.


Stan Weiner contributed to this answer. And did you know that he and Greg McMillan write a column each month in our sister publication Control Global? You can view the current Control Talk column at http://www.controlglobal.com/voices/mcMillan_weiner.html.