RTD and thermocouple

My friend was asked in his viva "How can you differentiate between an RTD and a thermocouple just by looking at it?" If anyone has an answer to this question i'd really appreciate it.

If in a sheath - a single T/C will have 2 terminals, while an RTD may have 3 or 4. But a duplex T/C will have 4 terminals. There is also often a visible difference between the lead wires for T/C (depending on type).

If the element is bare - a T/C will be a simple junction between 2 different conductor materials while an RTD will have a resistive element.

Bruce

By the color of the terminals. Usually the thermocouple has special colors, and only two wires.

If it has more than two, your job will be easy.

Mostly RTDs are 3 wired. And the size of the wire is small.

A TC will typically have (2) leads. A type J TC will have a white lead (+) and a red (-). You will be able to measure a millivotl potential across the leads.

A RTD will have at lease (3) leads (2) of one color and (1 or 2) of another color. The same color wires are conected at the sensor and act a resistance compensation leg of the sensor wiring. It can either be a single leg compensation type (3 wires) or both wires are resistance compensated in a 4 wire type. Lastly you will measure a resistance across the diferent colored wires. The value will be the basice RTD resistance at standard conditions plus the wire resistance.

In the USA, how would one distinguish between a 2 wire RTD (one red, one white) and a J T/C (one red, one white)?

Answer:
differentiate between an RTD & T/C is RTD is always in three wire system & is measured in resistance form.but thermocouple is two wire system & measured in voltage form.

A dual element T/C will have four leads--two of one color and two of another (they could be two white leads and two red leads). A four-wire RTD will also have four leads--usually two of one color and two of another (they could be two white leads and two red leads).

Isn't this one of those "trick" questions? Without using an ohmeter to measure the resistances of all the wires coming out of a temperature sensing device, one can't be sure what type of device it is.

Isn't the question trying to see if the responder will answer something like, "Without some indication like a label, it's not possible to be certain if the device is an RTD or a T/C; one should take an ohmeter to the leads to be certain if there is a question,"?

markvguy

Whatever arguments and counter arguments one may provide, there is no way one can confidently say it its RTD or vice versa.

But, yes we may guess if the options are limited like "Generelly the TCs are single element 2 leads and the RTDs are 3 leads with 2 of same colour"
Though one has guessed rightly, it can not be taken for application without confirming.

The simplest thing to do is put a magnet on the white(+) wire. If it is magnetic, it is a TC (J). If not, it is an RTD.

Using a magnet, or a multimeter is cheating, the poster said "just by looking at it". The RTD usually has 3 or 4 copper wires but if it has 2 one of the thermocouple wires will be stiffer. (I guess touching is cheating also.) If the wires are single strand, for sure it's a t/c.

Roy