PLC Scaling doesnt match Sensor Display Output

[PLCarp 0]

Hello,

 

I'm currently working with a 1769-L19ER-BB1B CompactLogix controller connected to 7 1734-IE8C analog input cards. Into these cards I have about 14 Setra 269 Differential Pressure Transducers, all of which have their own individual display. Our goal is to capture and record the data from the Setras in the PLC, but I am struggling to scale it correctly. I currently am running a series of SCL function blocks to change the raw inputs (3277 - 16383) to the Engineering range (-0.25 - 0.25). The Setras have a standard output range of 4-20 mA and the calibration sheet I have for it matches that (4 mA = -0.25, 20 mA = 0.25 etc.) My issue is the SCL function block outputs are not matching the displays that the Setras have on the sensors. The difference can be somewhat staggering (largest one ive seen is PLC - > 0.045 , Sensor - > 0.003. Any ideas on what I can try to fine tune this scaling? 

[BobLfoot 301]

Just an off the cuff idea.  What happens if you scale 3277-16383 to give you 0 to 50. 

If that works stabily then divide that result by 100 and subtract 0.25 to get your final value.

I am suspicious that the SCL Block doesn't handle the negative value well. 

[Joe E. 259]

Have you verified the mA signal matches the display? Also, verify the scaling in the Module Properties hasn't been messed with.

[PLCarp 0]

15 hours ago, BobLfoot said:

Just an off the cuff idea.  What happens if you scale 3277-16383 to give you 0 to 50. 

If that works stabily then divide that result by 100 and subtract 0.25 to get your final value.

I am suspicious that the SCL Block doesn't handle the negative value well. 

Good Idea. I tried this but I got the same value as just using the scale block. At least I can trust negative ranges in the SCL block for other uses now.

[PLCarp 0]

2 hours ago, Joe E. said:

Have you verified the mA signal matches the display? Also, verify the scaling in the Module Properties hasn't been messed with.

I'm beginning to think the same thing. The vendor reached out and said the displays arent calibrated only the output signal is. Working on getting a calibration key to zero out these displays and check to make sure 0 in WC = 12 mA signal. 

[DanW 41]

The vendor reached out and said the displays arent calibrated only the output signal is

Just a rant.  I've run into this on other instrumentation and it amazes me that a digital value for the measured process variable is available internally, so why isn't the digital display displaying the digital value and not converting an analog value?  You'd think that you could trust the local display as having the highest integrity as the representation of what's being measured, but no, the analog output is calibrated, not the display.