Answer / carl_ellis

Show me a 'grounded RTD'. I have never seen a commercial
grounded RTD. The probability of a measurement error due to a
ground loop is too high.

An RTD measurement requires a stable, constant current
source, which is generated by the sensing circuit. If an RTD
element were grounded, a ground loop could affect the current
source depending on the difference in ground potentials at the
RTD element and the sensing circuit. That would create an error
in the measurement.

Grounded thermocouples work some of the time, but other
times some form of isolation is needed because of a ground loop
issue with a thermocouple.

You use ungrounded RTDs because that's the best way for them
to function.

Carl Ellis
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