I know this isn’t a "thread on thermocouples, but----
I have written a bit about Thermocouples that I have used with my own woodgas reactor here:
This page has several real world examples, and offers hints on how to stay out of trouble when buying off of Ebay, etc…
For those who need to extend the length of the leads for a T/C they already have, there is a full range of:
“thermocouple extension wire” (google that) available. And there are several companies on the net that have very instructive websites,
https://www.omega.com/section/thermocouples.html is one of them.
Having said all that, I have found several sheathed, Type K T/C’s with “3 meter” leads. That, of course is about 9 feet, and so would not require any extension wire for most of our applications