Level sensing in a pressurized Vessel

I did an application 2 years ago where I need to know level in a pressurized tank and the system had an existing sight-glass that I took advantage of to use radar-level probes. I'm doing another similar project, but don't have the easy ability for the radar-level without having the tanks welded and re-certified. I am already going to have a pressure transmitter in the top of the vessel (for control), so I am wondering about the feasibility of using a pressure sensor for level and just subtracting the static pressure from the pressure (level) transmitter to give me my liquid column. The seems like it should work just fine.

I have seen this done with a DP transmitter where the two ports are top and bottom (or high and low) on the tank. I have only had my hands on the wiring, and not gotten into the details of how to set this up from scratch, but seems to be highly reliable.

Quick search found what looks like a good intro:
https://www.vega.com/en-us/company/...ement-101-understanding-differential-pressure

Depending on a lot of things, the difference between the two is likely going to be in inches of water. Your pressure transmitter is not going to have the resolution you need.

What is the MAWP of the vessel?

what do you mean by "static pressure?"


can you sketch the setup?

Differential Pressure (DP) for level has been used for probably 80 years now. It is a mature liquid level measurement technology.

A dual port DP transmitter is used in 99.9% of applications because the DP transmitter read fractions of an inch of water column DP at static pressures of 2000 psig. The high side port connects at the bottom, the low side port at the top of the vessel, and a tube/pipe is run from the vapor space above the liquid level down to the transmitter to the low side port to measure the working/static pressure.

Access ports can be pipe taps but a tank designed for a lower mounted DP transmitter will probably have a 3" flange at the bottom. The typical sensing diaphragm on a flange mount is a 2" diameter diaphragm (the wider the diameter, the more sensitive the pressure measurement).

Some limitations are

- level below the bottom tap can not be sensed, only liquid head above the bottom tap creates a readable pressure.

- head pressure is directly proportional to the density (Specific Gravity) of the medium. If the medium density changes, the change in density is a proportional error: 4.5% change in density is a 4.5% error in level.

- subtracting 2 gauge pressure measurements typically does not provide sufficient resolution. Rosemount has a set of two 'matched' gauge pressure transmitters that communicate between each other, one of them subtracting the low side from the high side to produce a level output. Technology is maybe 12 years old now.

- Temperature limitations on the diaphragm fill fluids (behind the diaphragm), specifically at vacuum pressure in the vapor space (spelled out in mfg specs)

- condensation in the tube/pipe running from the top side down to the transmitter located at the bottom can build-up and cause an offset error.


- liquid movement from an agitator or in/out flow can exert a force on the sensing diaphragm that is not hydrostatic head, an error in a level measurement.


Is there an access port at the bottom from which the pressure can be tapped? What's the working/static pressure in the vessel? Is the medium friendly (non-corrosive) to stainless steel? How hot/cold is the process?

Well I was going to respond with some information on DP level measurement, but danw has told you everything I know and then some. This is excellent advice.