We have rosemount 3051 dp cells ranged 0 mbar to 125 mbar. These are smart transmitters so only the hart values are used. I been looking through previous test sheets and these have sometimes failed their calibration. It was noted that that a zero adjustment was done or a zero and span adjustment was done. What exactly did they do? Do they do a zero trim for the zero adjustment applying 0 mbar and for span a low and high trim. Low trim applying 0 mbar then applying 125 mbar for high trim? Am a bit puzzled whether to use the zero trim or low trim if the zero is slightly high.

 

1) >What exactly did they do?

If you don't know what procedure your co-workers did, how would I or anyone else know?

2) I know you want a 3 sentence summary so I'll take a stab at it, but you should really need to read the 11 pages on calibration in the manual. Otherwise, it's just monkey-see, monkey do.

- Sensor trim adjusts what the transmitter's input sensor reports to the D/A

- Output trim adjusts the D/A's 4-20mA output

- Zero trim adjusts the reported DP at a given instant for a given installed condition at applied zero DP as "zero DP", which corrects for minor out-of-plumb-or-level, or an ambient temp condition.

The current 31051 manual is here, calibration is covered on paper pages 70 - 80; 80-90 pdf.
http://www2.emersonprocess.com/siteadmincenter/pm rosemount documents/00809-0100-4007.pdf

Note that there are HART commands to reset the transmitter to factory sensor (factory recall) and output trim conditions (analog recall), which, I would be inclined to do to eliminate the effects of previous unknown trims.

Kenneth L. Holladay wrote a paper on "Calibrating HART transmitters" some years ago. It is as valid today as the day it was published.
http://www.kudammcorp.com/sites/default/files/blog/CalHart.PDF

His HART transmitter block diagram helps to understand which trim function works where in a HART transmitter.
https://s31.postimg.org/eftod5fm3/HART_Transmitter_block_diagram.jpg

 

Zero trim or lower trim is often done at time of installation of the
transmitter to correct for any mounting position offsets you may see.

Zero trim is used when you apply no pressure i.e. 0 (zero).

Sometimes you can't apply zero (e.g. absolute vacuum) or you want to trim at a value other then zero for best accuracy maybe -10 mbar or +25 mbar or whatever the case may be. In this case you use lower trim instead, and you tell the transmitter the pressure you applying.

That is, zero trim and lower trim are very similar, only that zero trim is really for zero so you don't need to key in a value.

Lastly: don't confuse zero trim with setting lower range value. They are not the same.

See full explanation in the smart transmitter calibration tutorial found here:

http://www.eddl.org/DeviceManagement/Pages/Calibration.aspx

Cheers,
Jonas