Rosemount 3051 Dp level transmitter question

Hi guys. Was hoping someone could help me out with this. I did not install this, only was told to troubleshoot why the level isn't reading correctly. It is for an oil tank that is around 500 gallons. They are using a DP for the oil level measurement. It currently has LRV of 0 and URV of 48" of H20. The actual full mark is around 36". The problem is they put this vessel under a vacuum for eliminate the oil mist of the hot oil entering the tank. I', thinking that the LRV should be whatever the LP tap leg is seeing. A guy at my work said it shouldn't matter as it reads differential pressure. I don't think that is correct as the low pressure leg is going to be reading vacuum and the high pressure leg will not unless it goes completely empty. Could someone clarify this for me and help me to set up this transmitter. Personally I don't think a DP transmitter is the best for this application, but that's what they got. I used .90 as the specific gravity for the oil (just guessed) and came up with an URV of 40". Here is a quick diagram just to try and visualize what I'm working with. Thanks for any help. I'm not smart so that's why I'm here


Thanks for the sketch. It always help to have numbers. It would help to know what the direction or magnitude of the error is, like, the dip stick reads 32.5", but the transmitter reports 28.3", or whatever.

So the tank is 50" tall but is full at 36" with the DP transmitter mounted at the bottom of the tank. If there is impulse piping or tubing where you wrote "High Side" any elevation difference between the tank port and xmtr high side port should be reflected in the LRV.

I assume the pipe/tube from the vapor area above the liquid level is a dry leg, not a filled wet leg and that it is NOT a remote seal and capillary application, correct?

The transmitter reads a lower value in inches water column when the oil's Specific Gravity (SG) is below 1.00. To convert the transmitter reading to physical level, divide the transmitter's w.c. reading by the SG.

To determine the hydrostatic pressure of a column of oil with an SG less than 1.00, multiply the physical level by the SG to get the reading in water column.

Whether the vapor area above the liquid level is positive pressure or negative pressure (vacuum) makes no difference, the DP transmitter will correct for the vapor area pressure. A DP transmitter is essential for correct pressure/level measurement in a pressurized closed vessel, whether the pressure is positive or negative with respect to the atmosphere.

Example with only atmospheric pressure in the vapor area:
At atmosphere, with a level of 18" of SG 0.90 oil, the oil's hydrostatic pressure is 18"w.c. * 0.90SG = 16.2"w.c.
high side 'sees' 16.2"w.c. + 1 bar (atm)
the low side 'sees' 1 bar (atm)
DP value = high side minus low side = (16.2"w.c + 1 barg) minus 1 bar = 16.2"w.c.
16.2'w.c./0.9 = 18.0" physical level of oil.

Example with a vacuum in the vapor area:
At a vacuum of 0.75 barg
high side 'sees' 16.2"w.c. + 0.75 barg
the low side 'sees' 0.75 barg
DP value = high side minus low side = (16.2"w.c + 0.75 barg) minus 0.75 barg = 16.2"w.c.
16.2"w.c./0.9 = 18.0" physical level of oil.

The transmitter measures in units of water column (w.c.) so the receiver - the panel meter, PLC AI, whatever, is responsible for reporting the physical level value by scaling the 4-20mA input to correct for the specific gravity.

Incorrect scaling of the level value from the pressure value constitutes about 50% of all hydrostatic level issues that I run into.

LRV should be zero as long as there are no remote seal/capillaries, the transmitter is flange mounted on a nozzle at the bottom, or there is no elevation difference between the vessel's low tap and the transmitter's high side port. If the transmitter's high side port is below the vessel's low tap, then the elevation difference is the LRV value. For instance, if the xmtr is 5 physical inches in elevation below the low tap, then the LRV = 4.5"w.c. (5.0"w.c. * 0.90SG = 4.5"w.c.). With the offset of LRV at 4.5"w.c., the transmitter reports zero level as 4.0mA when the vessel is empty (but the impulse piping is not.

URV should be the highest physical level divided by the Specific gravity.
For a 36" maximum physical level for oil with an SG of 0.90, the URV would be 36"w.c.*0.90SG, or 32.4"w.c.

4.0mA = 0.0" level
12.0mA = 16.2"w.c. = (16.2"w.c./0.90SG) = 18.0" physical level of oil,
20.0mA = 32.4"w.c. = (32.4"w.c./0.90SG) = 36.0" physical level of oil,

If the URV is 48.0"w.c. then the transmitter could measure up to 48"w.c./0.90SG = 53.3 physical inches, a level higher than the 50" tank. A higher-than-possible URV should not create an error in the measurement, the transmitter will measure whatever hydrostatic pressure it sees, but it makes people scratch their heads trying to figure out why.

How is the URV of 48.0"w.c. (20mA) interpreted by your system? Is 48.0"w.c. (20.0mA) interpreted as 53.3" physical level?

Non-obvious problem with dry legs:
A build-up of condensate in the low side 'dry' leg will produce a constant hydrostatic head pressure on the low side, which will produce a false low pressure/level reading.

Any deviation from the specific gravity of 0.90 is direct proportional error in the interpretation of the physical level. A 1% change in SG is a 1% error.
Thank you for the reply and great explanation. The reading was low, about 12% level on the OIU, while the physical level in the sight glass was about 85%. It is a dry leg. It is not a remote seal and capillary. The low reading was verified by a hart communicator on the transmitter. The transmitter is mounted at the same level as the tap on the tank.
I realize that the DP transmitter should compensate for the pressure or vacuum in the tank as it will have an effect on both sides. Being in a vacuum though and not a pressure I wasn’t sure if the vacuum would have a greater effect on the low side tap versus the high pressure side that is covered by an oil level. Obviously pressure would have the same effect on the low side and the high side since the tank pressure would be added to the pressure caused by the level of the fluid. I couldn’t find anywhere in all of my searching that said if vacuum would work in the same manner.
This level reads in percent full on the oiu. It doesn’t have to be a highly accurate level, but should be close. Being as how the sight glass shows over 85% full and the OIU was showing 12%, that’s a little to much of a discrepancy.
1. > I couldn’t find anywhere in all of my searching that said if vacuum would work in the same manner.

It is not you, it's a universal deficiency in vendor documentation. It's assumed that 'everybody knows'.

2. Is the oil level absolutely, positively never going over that 36" physical level line? You don't want a buffer on the high end to let people know how much overfilled the vessel is?

If you want exactly 100% at 36.0 inches physical for 0.90SG oil, then

- LRV = 0.0"H2O = 4.0mA = 0.0% on the OIU
- URV = 32.4"H2O = 20.0mA = 100% level on the OIU (36" physical level* 0.90 = 32.4"H2O)

or allow for some overflow, where 36.0" physical level is still 100%, but with some overfill buffer :
- LRV = 0.0" H2O = 4.0mA = 0.0% on the OIU
- URV = 45.0" H2O = 20.0mA = 139% level on the OIU (50" physical level * 0.90 = 45.0"H2O) 50"/36" = 1.39 = 139%
- LRV = 0.0" H2O = 4.0mA = 0.0% level on the OIU
- URV = 40.5" H2O = 20.0mA = 125% level on the OIU (45" physical level * 0.90 = 40.5"H2O) 45"/36" = 1.25 = 125%

Nothing says that 20.0mA has to be exactly 100%.