Using the "bias" parameter on a AB 1771-IR RTD card

A customer of mine is having problems with the accuracy of an RTD that is wired into an AB 1771-IR RTD card. The RTD is off by 5.2 degrees F at 32 degrees (37.2). I replaced the RTD with a new one and had the same result. I moved the wires to a new point on the card and got the same reading.

After doing some research, I found that if the resistance on the lead wires on a three wire RTD are not matched, it can cause an accurracy error. I believe this is the case.

My question is this: Instead of replacing the wiring, my customer would like to use the bias parameter on the RTD card configuration to correct the reading. Is that what the bias parameter on the 1771-IR is used for?

I cannot find any documentation that describes the function of the parameter in any detail.

Also, if you know of a good paper on the proper installation and wiring of RTDs that would be a big help.

Thanks

Try this web page:

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1771-um668_-en-p.pdf#xml=http://127.0.0.1/texis/search/pdfhi.txt?query=1771-IR+bias&pr=literature.rockwellautomation.com&prox=page&rorder=500&rprox=750&rdfreq=0&rwfreq=0&rlead=250&sufs=1&order=r&cq=&id=42d9cc51a

The bias setting (in BCD) is subtracted from the Channel data. So, if you want to give a shot, enter 5.2 into the bias and recheck at 32, the input value should be 37.2-5.2=32, you should also check at some other value to see if the offset is the same through out the range of the RTD.

If you know the guage and length of the wire you can calculate the temperature shift it causes. Just figure the resistance out and back, and go to a RTD resistance vs. temperature table, add it to the resistance for the expected temperature, and look up the new temperature. If worst comes to worst you can deduct the increase in your logic rather than configure the card.