Rtd Wiring Considerations - Spectrum Controls 1769 User Manual

2-12 Chapter 2: Installation and Wiring
NOTE
NOTE
User's Manual Pub. 0300241-02 Rev. A

2.8.2 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.
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.
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 ensure temperature or resistance value accuracy, the resistance
difference of the cable lead wires must be equal to or less than 0.01Ω.
To ensure 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, and
consistent impedance throughout its length.