Answer / carl_ellis

An offset in readings from separate RTD's can indicate
- a difference in calibration of the analog input devices
- a bad RTD element, one that has drifted out of spec
- a common ground issue with ground loops between the analog
input devices

An RTD is supposed to be ungrounded and the two RTD's in a
duplex element are supposed to be electrically isolated from
one another, so it is unlikely the ground loop is unlikely
unless poor wiring of the extension wire or head wiring has
created a connection to ground.

If you're complaining about a small difference of the
indicated temperatures, then get out your cal gear and cal
the inputs.

To check for a bad RTD, I'd take an ohmmeter and see if any
leg on the A side has any connection to the B side by
disconnecting the wiring from each analog input and check
the resistance. If there's a low resistance, it indicates
some connection from A to B. If the extension wires are
then disconnected and the resistance is checked on the RTD
lead wires and the resistance remains, then it's a bad RTD.
If the resistance goes high (open circuit) then the
extension wire is shorting or making a low resistance
connection somewhere.

Carl Ellis
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