The two metals that make up a thermocouple wire run the entire length of the thermocouple right? (As opposed to an RTD where only the tip is the temperature sensitive metals and regular copper wires lead away from it).

So if I wanted to shorten a really long thermocouple, I just cut the tail end wires off? It doesn't matter that the tail wires are not made of copper but are instead made of the same material as the junction? It has no effect on the performance of the thermocouple?

 

Yes and no.
A thermocouple makes a voltage based on the difference between the hot junction and where the thermocouple wire is mated to copper wires (the cold junction). Not an absolute temperature. Normally we measure the cold junction temp with a simple thermistor or just assume it to be like 75F, and infer an absolute temp of the measured temp from that.

If the wires are very short, the wires may be conducting heat off the hot junction. Taking a thermistor measurement next to the cold junction won't compensate for it, because the thermistor wire's metal itself is hotter than the ambient air around the cold junction.

Also if there is no cold junction compensation, a short thermocouple wire run may place the cold junction in a location which is heated by whatever hot area you're trying to measure. This makes the assumptions about ambient temp inaccurate.

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I thought what I'd do was I'd pretend I was one of those deaf-mutes.

 

I am using the Maxim Cold Junction COmpen IC (thank goodness for magic black box ICs!). So you are saying, just keep the wires long enough so that there is a temperature difference between the hot and cold ends, where the cold end is at ambient temperature (the same as the compensation thermistor temperature)