RTD VS thermocouple

Thermocouple will have 2 wires of different alloy, identifiable by insulation color. RTDs have 3 or 4 wires, and will usually have 1 or 2 wires of one color and 2 wires of another (usually white/red). If it's an RTD, you can measure resistance between any 2 of the different colored wires. If an RTD, the white wires are connected internally as are the red wires, which you can test also with a multimeter.

Brad Waybright

The more you know, the more you know you don't know.

IF you're in the automotive world, you might run into thermistors, too, whose resistance changes with temperature. Thermistors are always wire devices. Some a negative temp coefficient, some are positive temp coefficient.

The junction of a good thermocouple will measure about an ohm but thermocouple wire has fairly high resistance compared to copper, as much as 0.6 ohm per double foot. Since a thermocouple generates an EMF across its temperature gradient, a resistance measurement with a DVM, which generates a current to measure the voltage drop across the probe leads for a resistance measurement, can be affected by the T/C's EMF. But unless there's a long stretch of thermocouple lead wire, then overall resistance is likely to be much lower than any commercial RTD (commonly 100 or 1000 ohms) or a thermistor.

There are 2 wire RTD's out there, which always amazes me, but they're there.