RTD VS thermocouple

[helpyou]

Hi

How can you identify from field if installed temperature sensor is RTD or thermocouple.. is it from wiring or something else? Your comments from experience please

Thank you

 

[thebard3]

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.

 

[danw2]

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.