To measure the liquid in a pressure vessel, you would need two connections. One below the liquid level and one that will always be above the level that will compensate for the vessel pressure. If you are not using remote seals, the transmitter would need to be mounted at or below the lowest measured point.
You can, but it will not be reliable unless you can you can ensure the lower sensing line contains a consistent amount of gas and liquid. A changing combination of liquid and gas in the lower leg will throw off the calibration.
One way I have made this situation work reliably is to introduce a small flow of gas into the lower sensing line near the transmitter. This keeps the tube clear of liquid, and works just like a bubbler. Make sure the purge gas flow rate is low enough to prevent frictional losses from changing the reading. A side benefit of this is that liquids that tend block the sensing line are kept out of the line.
Recommended location is at the datum level (zero level of vessel) or below the datum level. Above the zero level will result shift in the actual zero of the vessel.
Even if you backfill and purge all of the air from the line most liquids have some entrained gas that will build up in the sense lines over time and cause problems with error and process noise on the signal. You can probably use diaphragm sealed and filled sense lines if you can work out the connections. But be careful with the fill fluid because there will actually be a vacuum from the transmitter relative to the lower process tap points. If the vapor pressure of the fill fluid is too low, then that fluid may actually "boil off" and leave you again with a gas in the sense line.
