One thing I’ve learned in this industry: Even though there are standards to thermocouple color codes and types, the most basic rule for installation (In the US, when you’re wiring a thermocouple, RED is always negative) is non-intuitive to anybody who’s ever done electrical wiring.

It’s pretty easy to tell when you’ve done it wrong: You wire the thermocouple directly into an instrument, and as the process gets hotter, readings say it’s cooling down. As your process cools down, the readings show a rise in temperatures.  If you reverse the wiring at a junction box, it’ll read in the right direction, but you’ll have errors because of the false junctions.

Here are some basic wiring diagrams from the reference section of the Lesman catalog, with rules to follow, and some suggestions on specifying the right thermocouple wire.

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