I just started as an automation tech about a year ago and have a question about a Rosemount 3051 DP transmitter used as a level trasmitter ranged 0-105" in H20. It is reading about a foot lower than what is in the sight glass next to it, but the transmitter is mounted about a foot lower than the zero of the sight glass. Is there a setting to put like an offset using the 475 handheld or is there something else i would need to do?

 

There are terms like Zero suppression and elevation just go through that...

have u mounted level transmitter and LG on same level bottle / same trappings.

Then isolate it from process drain level glass.. and see what pressure reading on transmitter and apply it as LRV and by adding ur span range apply it as URV. from HART

Or by draining level bottle press zero switch on ur level transmitter.

 

So if I go through the zero suppression that would be like me rearranging it from 0-105" H20 to 12-117" H20 since the level is showing 12" lower than what it actually is in the sight glass?

 

Yes. 12-117 in wc.

 

Ok let’s get you on the right track. I will give you a detailed explanation on how to setup any dry leg DP transmitter making use of S/S impulse lines on an open atmospheric tank or a closed pressurized vessel. Also take note that this is for a tubing transmitter only since working with chemical seals and capillary tubes on a DP transmitter is something else, don’t try to use this procedure on those, it will not work. Also take note that setting up a dry leg on a closed pressurized vessel is not the best of ideas but if you have investigated everything and still feels that this is the right way to go, this is how you do it.

Ok you say your transmitter is installed about 30mm below the zero of your sight glass. I prefer to work and take my measurements in mm, but if you prefer inches, no problem, the procedure stays the same.

Start by taking precise measurements with a measuring tape, first from the point where the tubing is connected to the DPT on the LP side, to the point on the vessel where your zero must be. Normally this will be at the bottom of the sight glass but it can be anywhere on the vessel. If it is not at the bottom of the sight glass, remember to mark the same point on the sight glass where the zero is.

Next take a measurement from this zero mark on the vessel to the point where you want span or 100% to be. The first measurement from the transmitter to the zero point on the vessel is called “h” and the one from the zero point to the 100% mark is called “H”.

Once you have these measurements use the following formula to calculate your LRV and URV which you can input as is with the 475 HC.
LRV = sg x h
URV = sg x (h + H)

In this case my answer will be in mmH2O but if you have used inches your answer will be in inH2O so make sure that the units of measure (UOM) are set correctly in the DPT.

Once this is done, put the DPT on line and have a look if a discrepancy still exist between the sight glass and the DPT reading and if so make small adjustments to the URV until the discrepancy is eliminated.

Leave it for an hour and check it again and again make small adjustments to the URV until you have eliminated the error. Remember a zero adjustment will affect your span, but a span adjustment will not affect your zero, therefore don’t do any adjustments on the LRV during this fine tuning stage.

You can isolate end open the transmitter to atmosphere again and do a zero trim with the 475 and then put it back on line to ensure that the zero is good if you have made a lot of URV adjustments, but don’t make adjustments to the LRV while the transmitter is on line. To ensure that a transmitter will work properly always spend most of your time on the zero and make sure that it is and stay accurate. Setting up the span is the easy part.

What you further need to understand is that the above procedure is suitable only for a dry leg installation on an open to atmosphere tank or for a closed pressurized vessel at around ambient temperature. At higher temperatures most products produces condensate which in time will fill up your LP leg which will cause your transmitter to lose its calibration. A dry leg on a pressurized vessel also needs to be drained at least once a week. Bottom line is stay away from dry legs unless it is on an open vessel.

Also just for information you can do this calculation in the w/shop and then take your HART Communicator and just input these values into the 3051 without a pressure pump. The 3051 is a SMART transmitter and will tell you if something is not right so you don’t ever need to pump a SMART transmitter up during a calibration. Good luck and give us a call if you get stuck again; there is always someone out there with an answer to a problem.

 

Yes. Re-range the zero. Learn more about re-ranging from the calibration tutorial found here:
http://www.eddl.org/DeviceManagement/Pages/Calibration.aspx

Cheers,
Jonas