Diff Pressure xmtr location in Liquid CO2 Tank

Tell me if I'm all wet here because I'm arguing with PE but I'm not yet convinced.

I have a 20 foot tall liquid CO2 Tank with vacuum jacket. The Level transmitter for this tank is mounted almost 3 feet from the bottom of the tank and its connections run down the side and into the bottom where I have to assume they connect to the top and bottom of the internal tank

I believe that I will be unable to accurately measure below the level of the mounted transmitter. i.e. nothing under 3 feet. The Engineer says that is not the case and it will work fine mounted there.

Since pressure in the tank will be equal, wont the column of liquid seek level at 3 feet ?

After writing that sentence it also occurs to me that there is no way that that cryogenic liquid is piped outside the tank against the diaphragm of that transmitter. There's no insulation on the lines. I wonder what does actually contact that transmitter...

I have added these to horizontal tanks and found this assumption to be true so what in this application am I'm missing here?

Based on your original description you are correct - if the transmitter is mounted 3' above the tap into the process fluid then you will get a zero reading at 3' level in the tank. For reference, look at how a manometer works! That assumes that the top of the DP transmitter is connected to the top of the tank. If it is open to atmosphere you'll measure the gauge pressure in the tank at the 3' level.

If there is a diaphragm then you have to deduct the pressure created by whatever fluid fills the 3' tube.

Without a good diagram it is hard to be definitive. Just remember what Daddy used to say - water seeks its own level.

Seeking level... that's the idea I am working under as well. The tank will be under ~400 psi so I'm reasonably certain both ends of the DP gauge are in that vessel.
Now if there a diaphragm in the bottom of the tank and the tube to the DP is just gas of whatever type, would the same problem exist? IN this case there's no liquid in the tube... Hmm.

sorry tom we are talking about CO2.
the pipes may be isolated it does not matter.
The trick:
in the pipes is only gas never any liquid, as the heat from outside will evaporate the liquid inside. (however you should worry when the line freezes up completely)
the pressure in the bottom side equals the level in the tank, the top is just for reference so the difference between both is the liquidpressure thus the level in the tank.

I do this in fishtanks, by blowing air in the bottom line so the water is pushed out, thus the pressure in this line is the liquidlevel.

The idea that there may be a diaphragm of some sort withing the tank might make this possible, but I can see a real problem should that side of the transmitter tubing fail or I have to replace that transmitter, I would then need to re-pressurize that tube to prevent the diaphragm over extending. And there would be no way to know what to pressurize it to.

1) Is the lower mounting point 3 feet above the bottom of the tank or below the bottom of the tank?

2) Does the transmitter have either one or two capillary and diaphragm seals, or is it just a plain DP transmitter with threaded ports?

3) A diaphragm would have to part of a 'diaphragm seal'; integrated with the transmitter via a filled capillary tube in order for the transmitter to make a reasonable pressure measurement.

A diaphragm on the tank itself would not provide suitable pressure measurement.

1000 Words...

Heres a pic

I have seen the fluid filled diaphragm type and this is not that.

Here is the diagram.

Looks like the tap goes to the bottom of the tank?

Yes it does.

The tap location is what matters not the transmitter mounting location. As shooter mentioned you are reading the change in gas pressure in the tube.

I think the key here is that the tank is under pressure. So the bottom tap sees tank pressure plus liquid pressure, where the top tap sees only tank pressure. The difference will always be the same (tank level), regardless of tank pressure (or instrument mounting location). When the tank is filled, the pressure relief lifts a time or two until pressure stabilizes at an acceptable value. While that is happening, the level indication does not vary.

You're all wet. The engineer is correct...as always.

So would that be true if the gas in the tube was bled and the process fluid was allowed to fill the tubing all the way to the transmitter?

Might be. I can convince myself both ways on that argument. Take a DP xmtr to RTPS and do the experiment - if you see it you'll never forget it.

I have tested this on a horz tank and with water contacting one side of the transmitter the location of the transmitter made all the difference in the world. Now CO2 will always be a gas outside that tank so I can envision that working differently... maybe.