Calibrating DP-Type Level Transmitters for Cryogenic Tanks

the HIGH side pressurized with a calibrated pressure source

Hmm, does your "calibrated pressure source" have a calibrated pressure gauge that will show you the actual pressure it's putting out? At least that way, you'd know what pressure you're injecting into the High side with respect to the atmosphere which is what your Low side has.

Further, is your transmitter at the same level as the lower tap? If it's higher or lower, your zero will change when you reconnect the transmitter to the tank.

Vulcan:

Many thanks for the tips--my pressure source is indeed equipped with a readout, and is calibrated to NIST specs. I had not paid attention to potential elevation/depression offsets, however, and will take that into account at my next calibration (my initial instinctive response is: I need to do more research. These are very tall, vertically-mounted tanks, with capacities of 26,000 lbs for the LOX and 52,000 lbs for the CO2, and I think that the vapor-side inlet is physically higher than the liquid-side tap).

Again, thanks--Toltec

As Vulcan said, pay special attention to the physical location of the dp sensor and routing of the sening lines. Fully understand the impact of liquid head. Where will the cryogenic liquid be vaporized in the sensing line if using direct sensing? If using capillary lines and diaphragms, remember to account for liquid head in the capillary.

I presume all fluids are saturated so temp and pressure are not independent. As such, the density of the liquid may change significantly with temperature / pressure. In this case, a true level sensor and d/p cell will not react the same to a change in density.

Manufacturers will say a d/p cell is not sensitive to absoulte pressure, and in my experience, this is true.

You may also need to pay special attention to the temperature ratings of the d/p sensors. If the sensor is exposed to the cryogenic temps, it may exceed the temp rating of the instrument. You can protect agains this by a well designed sensing line, but always know the impact of liquid head in the sensing line.

Worksalot:

Thanks for the tips--unfortunately, I do not have report back on the temperatures inside the tanks, only the vapor-side pressure, but I am fairly confident in those readings (the insulation quality of the vessels is inspected/verified annually). I will certainly review the temperature specs of my d/p sensors. I am unfamiliar with determining the effects of cryogenic liquids in the sampling lines (especially with where they may be changing density), but I have been curious as to whether the instrument sample feed lines, being exterior to the tanks, uninsulated, and exposed to varying ambient temperature (about 10' or so of total tank-exterior lines) could be contributing to my post-calibration inaccuracies. I've got some research to do!

Much obliged to you and Vulcan for your help--Toltec.

Be careful to choose calibration range. When you are measuring the cryogenic liquid you have to pressure the density based on possible pressure the tank is likely to operate. If saturation pressure varies so does the density and equivalent mmWC will change based on that