# How to measure tank level through differential pressure

J

#### Joe

Hi to All,

I understand that differential pressure reading should be proportional to that of a tank level. The element inside the tank is CO2. What I did is to scale 0-35 bars (DP range) to 0%-100%. Or do I have to have further calculation considering density and tank measurement?

I would appreciate for your help.

Thanks,
Joe

D

#### d

d/p = delta L[m] * density[Kgf/m3]

for d/p in Kgf/m2

likely that you transmitter is spanned in mBars not bars

the complete calculation includes the wet leg fill density, and correction for the vapor space density.

your transmitter supplier usually has calculation methods written up

#### David_2

Presumably this is liquid CO2 in a pressurized vessel?

You probably have the wrong DP pressure transmitter if you can scale its output to 35 bar differential. The DP transmitter does have two ports, a high side and a low side, right? (a single port is a gauge or absolute pressure transmitter) A typical DP transmitter used for tank level measurement would have an output range of approximately 1 bar differential

Liquid column for level measurement is measured by a DP transmitter in units of water column: mm, meters or inches (maybe feet) of water column.

DP transmitters are capable of internally measuring static pressures in the 35 bar range, but that's the static pressure (the pressure of the gas blanket above the liquid level), not the differential which is the liquid level range measurement.

A liquid level of 35 bar would be a liquid column of water 356m (1170 feet) high. Is that the measurement you're looking for? I doubt it.

I suspect you're talking about an insulated storage tank, with a liquid level range most likely in the several meters or several feet range, right?

If your output range is truly 35 bar differential, you need a different model DP transmitter because a transmitter ranged 35 bar differential will not resolve a level of only several meters of liquid.

A DP transmitter subtracts the vapor blanket pressure it sees at a port or tap at the top of the tank (connected to its 'low' pressure port) from the pressure it sees at the bottom of the tank (connected to its 'high' pressure port) which is a combination of both vapor pressure and liquid column pressure.

(high side pressure) minus low side pressure = liquid column pressure

(Vapor pressure + liquid column pressure) minus vapor pressure = liquid column pressure

Supposing the tank vapor pressure is at 17 bar (approx 250psig).
(17 bar + 1.75m liquid level) minus 17 bar = 1.75 m liquid level

The DP transmitter output is then 1.75 m (water column)

But 1.75m is the level of a column of water, not a column of liquid CO2 because CO2 has a different density than water.

To convert the water column measurement to physical liquid elevation, the water column value is divided by the specific gravity of CO2 (at temperature). The result is mm, meters or inches of liquid CO2 at that temperature.

The resulting value in mm, meters or inches (whatever the transmitter is configured for) is the physical tank level above the bottom tap (assuming the DP transmitter's high side port is horizontally level with the bottom tap).

J

#### joe

I thank you so much guys for your answers. The range is 0-2.8 bars by the way. The supplier gave me a calibration data with mmWC, Volume (cubic meter), and weight corresponding figures.

Am I right that the DP has been customize to this tank with respect to its measurement, and of course to CO2? Basically, they just asked me to read the DP in percent, and that's already the level measurement.

I had experience in level measurement but through radar instrument which is straightforward, but never had experience through this differential pressure.

Again, thanks.

Joe

#### David_2

I haven't a clue what your supplier actually provided. You need to ask them to be very specific about what they've delivered to you.

One would hope that they've configured it for 0-100% output, density compensated for your specific tank dimensions, but who knows?

K

#### Karol

Hi Joe,

I'm struggling with choosing the right CO2 sensor for my system. Could you tell me what kind of sensor did you use and what are your experiences with it?

Thank you