1756-IF16 difference in mA reading

Hi All,
I have 4 PDIT (HART) transmitter attached to 1756-IF16 AI card as single ended float.
The PDIT (Rosemount 3051CD) is ranged from -1 to 1 (re-scaled from -3 to 3) in of H2O and generally reading around 0.0023 inh of H2O in operating conditions and the customer require it upto 3 decimal.

here is what is happening.
I have my fluke meter in series. I scaled the channel to read 4-20 engineer units to mimic 4-20mA readings.

Using the Hart communicator, I can see that the PDIT is reading 11.546mA (-0.0566 in. H2O), my meter reads 11.548 mA and PLC reads 11.987mA (-0.055 in.H20) and there is similar trend on all 4 PDIT.

However, When I disconnect the transmitter and use my meter as a simulator, if i force 11.548mA, i read 11.548 mA in my PLC.

so not sure why i am seeing the difference in mA signal when the transmitter is connected.

Would appreciate any input and solution to get over it.
Thank you.

Your meter and the Hart seem to agree to about 2 decimal places FOR MILIAMPS, which is pretty good. The PLC 1756-IA16 analog input module probably doesn't have and will never have the resolution to get 3 decimal place accuracy in miliamperes.

So with what you have, you can give the customer a 3-decimal-place data display, but you can't give them accuracy to 3 decimal places reading in miliamperes. However, reading in inches of water, you just may have it already because the typical 0.002 reading is 3 decimal places. Cut if off at 3 places so you don't display the error at 4 decimal places in inches of water.

Hi There,
Thank s for your reply.
But I am puzzled by the fact that if i use my fluke meter to simulate 4-20 mA signal with precision upto 3 decimal i am able to read it in my PLC

It could be noise on the line that the input module is sensitive to and the Fluke isn't. Put a scope on the signal.

There is also a specified gain drift on the module of +/- 0.41uA/Deg C, is the value stable? or does it change? It could just be caused by the ambient temperature of the module.

How do you have the module connected? 16 Channels? Differential? High speed?

Lancie1 said:

Your meter and the Hart seem to agree to about 2 decimal places FOR MILIAMPS, which is pretty good. The PLC 1756-IA16 analog input module probably doesn't have and will never have the resolution to get 3 decimal place accuracy in miliamperes.

Click to expand...

I thought the same thing so I looked up the specs. That input card is 16 bit over a range of 0-21mA. That's 0.00032 mA per bit. Not too shabby.

If the hart device is supplying power to the loop maybe it does not have enough to drive both the meter and the analog input.