Scaling to prevent "Uncertain" or "Bad" status

[solodiver2]

We have competed our first Fieldbus installation on DeltaV but are having a couple of issues with scaling.

Any would be appreciated.

We have some Rosemount 3051 pressure transmitters.
They are calibrated 0-25 in H2O @68. Our XD_SCALE and OUT_SCALE are also both 0 - 25 in

We are getting an "UNCERTAIN" status when the system is not running because the pressure is sometimes slightly below zero. We don't have this problem with our standard I/O because "UNCERTAIN" starts at -10% and "BAD" at -20%

How should I resolve this problem?

Should I change the XD_SCALE and OUT_SCALE to go below the process
range for example ( -1)?
If so is it okay to have the XD_SCALE slightly outside the calibrated range or should they be re-calibrated (-1 to 26)?

Do you have a "standard" way of dealing with this type of problem?

[rezabejd]

This "feature" is among my pet peeves. See this thread (http://forums.fieldbus.org/showthread.php?t=479):

http://forums.fieldbus.org/showthread.php?t=479

I would like to see this "feature" removed. I want my DP cells to keep indicating when the raw measurement exceeds XDSCALE. Why neuter one of the many benefits of digital signals and digital integration of field devices?

Who wanted this, or thought users wanted this? What was the justification, I wonder? We started out with ITK3 instruments and were blissfully free of this nuisance, until we started buying and installing later models.

The work-around (and that's precisely what it is) is as you suggest . . . figure out what negative 1 inch is in your flow units (which is tricky for square root measurements) and also make sure you're covered for readings in excess of 25 inches - if you anticipate running there. The sub-range and over-range flow extremes, corresponding to your xdscale extremes, need to be entered for out_scale . . .

Don't worry about recalibration. Just zero the instrument at operating static pressure like you normally would.

John Rezabek
ISP Industrial Chemicals

[solodiver2]

Thanks for the insight. I am not an instrument guy and installing fieldbus is forcing me to userstand more than just 4-20mA.

With fieldbus instruments am I even gaining anything by calibrating my transmitter from 0-5 inches instead of the full factory range of 0 to 25 inches? If everything is already digital it seems I am just using the first 6 inches of the same yardstick.

Mike

Washington Savannah River Co.

[rezabejd]

Mike,

Looking at the accuracy specs for a 3051S, they are stated in % of span for non-fieldbus and % of calibrated range for FF. Static pressure and temperature effects (as I recall, they are bigger actors in total uncertainty) are stated in % of URL plus % of span.

In that respect, if ISO9000 types or similar consultants / regulators are seeking "manufacturers stated accuracy", there may be some value in having a calibrated range that better matches your specific application.

I'm with you though . . . I think it's the same "counts per EU" whether you're using 10% of the URL or 100%.

John R
ISP Lima USA

[solodiver2]

Do you have any experience with Rosemount 848Ts. Will I have the same issues if my XD_SCALE is too small?

[rezabejd]

I have about 10 or 12 848T's and haven't seen it yet.

[solodiver2]

I tested an 848T Rev 4 Thursday and it didn't show "uncertain" until it was outside the max range. The XD_SCALE made no difference!