Rockwell Automation 1769-IR6 Compact I/O 1769-IR6 RTD/Resistance Input Module - Manual (Page 37)

Rockwell Automation Publication 1769-UM005B-EN-P - March 2012 37
Installation and Wiring Chapter 3
RTD Wiring Considerations
Since the operating principle of the RTD module is based on the measurement
of resistance, take special care when selecting your input cable. For 2-wire or
3-wire configurations, select a cable that has a consistent impedance
throughout its entire length.
When using a 3-wire configuration, the module compensates for resistance
error due to lead wire length. For example, in a 3-wire configuration, the
module reads the resistance due to the length of one of the wires and assumes
that the resistance of the other wire is equal. If the resistances of the individual
lead wires are much different, an error may exist. The closer the resistance
values are to each other, the greater the amount of error that is eliminated.
To insure that the lead values match as closely as possible:
· Keep lead resistance as small as possible and less than 25 Ω .
· Use quality cable that has a small tolerance impedance rating.
· Use a heavy-gauge lead wire which has less resistance per foot.
Terminal Door Label
A removable, write-on label is provided with the module. Remove the label
from the door, mark your unique identification of each terminal with
permanent ink, and slide the label back into the door. Your markings (ID tag)
will be visible when the module door is closed.
IMPORTANT The RTD module requires three wires to compensate
for lead resistance error. We recommend that you do not
use 2-wire RTDs if long cable runs are required, as it
reduces the accuracy of the system. However, if a
two-wire configuration is required, reduce the effect of
the lead wire resistance by using a lower gauge wire for
the cable (for example, use AWG #16 instead of
AWG #24). The module’s terminal block accepts two
AWG #14 gauge wires.
IMPORTANT To ensure temperature or resistance value accuracy, the
resistance difference of the cable lead wires must be equal
to or less than 0.01
Ω .