RTD or thermocouple: choosing a temperature sensor
Two technologies with different failure modes, and a straightforward rule for which to use.
Short answer
An RTD measures temperature as a change in resistance and is more accurate and stable over a moderate range. A thermocouple generates a small voltage from two dissimilar metals and covers a far wider range, including very high temperatures. Use an RTD below about 600 degrees where accuracy matters, and a thermocouple above it or where response speed matters more than precision.
Two ways to measure temperature, with different physics and different ways of being wrong.
RTD
A resistance that changes predictably with temperature. A PT100 reads 100 ohms at zero degrees.
More accurate, more stable over time, and limited to roughly minus 200 to 600 degrees. Slower to respond because there is more mass.
Thermocouple
Two dissimilar metals joined, generating a small voltage proportional to the temperature difference between the joint and the other end.
Wider range, up to 1,700 degrees for some types, faster response, cheaper. Less accurate and drifts over time.
The rule
Below 600 degrees where accuracy matters: RTD. Above that, or where speed matters more than precision: thermocouple.
The failure modes differ, which matters more
An RTD fails open, which reads as maximum temperature on most cards. A thermocouple failing open also usually reads high.
Both fail towards hot, so a burnout detection setting that trips an over-temperature alarm is doing its job. Configure it deliberately rather than discovering it during a fault.
Cold junction compensation is the thermocouple problem that catches people. The terminal block where the thermocouple meets copper is itself a junction generating its own voltage, and the card compensates using a local temperature sensor. Put the terminals somewhere hot and the compensation is wrong.
Common questions
- Why does my thermocouple reading drift with ambient temperature?
- Because the junction where the thermocouple wire meets copper generates its own voltage, and that has to be compensated. If cold junction compensation is misconfigured or the terminal block is in a hot cabinet, the reading follows the cabinet rather than the process.
- Should I use 2, 3 or 4 wire RTD?
- Three wire for most industrial work: it compensates for lead resistance and is the usual compromise. Two wire is acceptable only for very short runs, because the lead resistance adds directly to the reading. Four wire is for laboratory accuracy.
Keep reading
- Safety
SIL or PL: which one does your machine need?
Two standards, two scales, and one machine. Which one applies, how they map to each other, and why the answer is usually ISO 13849.
- Safety
Categories B, 1, 2, 3 and 4, in plain terms
Five architectures, what a single fault does to each, and the practical wiring that goes with them.
- Safety
Safety relay or safety PLC: how to decide
One is a wiring decision, the other is a programming one. The count of safety functions, not the size of the machine, is what settles it.