RTD & Thermocouple

Doesn't have to be. One could use two, three or four wires for either thermal transducer. It is all a question of precision and reliability.

...which is not an issue with a thermocouple, as it produces a milllivolt signal that only uses two wires.

See this description of an RTD which includes the rationale for the third wire

http://www.omega.com/temperature/z/pdf/z033-035.pdf

The thermocouple works on the principle of different metals in junction produce a voltage when one end is at a different temperature than the other. The "wiring" of the thermocouple is actually the 2 different metals. Hence the 2 wires.

The RTD's are a temperature driven variable resistance device that varies resistance with temperature. Two wires are all that is needed here, but we throw in a third or fourth as a way to allow compensation for the wires resistance.

The RTD has an ambient temperature compensation circuit, and the thermocouple not.

Not good. I was still pondering the 6" vs 8" stove pipe problem.

The issue is the wheatstone bridge employed in the circuit, which is not the case with the t/couple, which has a known temp versus voltage value.

Actually, it is a thermocouple measurement that has the 'ambient compensation circuit', more commonly known as cold junction or ice point compensation.

The temp vs voltage table values are only valid for ice point compensated measurements.