Interesting article here
http://circuitcellar.com/cc-blog/the...thermocouples/

The thermocouple measures difference not absolute temp.

There is a hot junction (measurement end) and a cold junction (at terminal block).

The difference is read as voltage. The terminal block is assumed to be at zero degrees.
In real application a chip is needed to measure terminal block temp and subtract it from the reading, thus making the reading more accurate.

However for this to work, it assumes the chip and terminal block are at the same temp.

Looking at the mega board cooling design, it's clear that hot air from the stepper drivers is driven over the chip and junction block.

This may be creating a difference in temp between chip and junction block.

I plan to test this theory out later today.

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