Thermocouple Drift.

I'm using a heater meter with a thermocouple (from thermoworks). I've done a bunch of cooks with it happily.

Yesterday, during a 12 hour cook, the thermocouple started drifting up high; 30-50 degrees high. It doesn't jump up, it drifts up (looking like the pit is getting hot). After a while (some number of minutes, seems to vary), it will drift down again. At one point it drifted high enough long enough it almost put out the fire; when it drifted down again, it actually triggered the lid open heuristic.

A second thermistor in roughly the same spot showed steady temps, and sneaking a thermapen in there showed the significant temperature difference (and agreed with the thermistor more or less).

I have enabled the ac filtering option.

The day was very wet and rained on and off; there seemed to be no correlation with the readings, and the heater meter (and PS) was protected against the rain.

Any ideas where to troubleshoot? I want to smoke a brisket next weekend but have lost confidence (maybe I can reassign one of the other inputs as being the pit?)

Here is a graph. The two vertical lines are 2 hours. The orange line is thermocouple; the green line is thermistor right beside it. You can see the thermocouple bouncing up and down, and reading somewhat consistently high (though varying a lot in how much!)

I know if you disconnect the TC amp from the ATMega the TC graph will rise up like you see there, so perhaps check the 100K resistor that stands up behind the TC jack for loose solder.....

Thanks ralph. I'll give it a really good look.

Ran a boil test with 2 thermistors and the thermocouple, all checked against the thermopen. Other then needing a little offset, they all behaved consistently and perfectly. Sigh. Only reproducible when meat is on...

Would an undersized 12 supply cause this? Just realized I'm running a 12V 750mA brick (supposedly) with a PI 1. I've not seen any PI reboots or anything, and it tends to be sensitive, so I doubt it is undersized, but thought I'd ask. Analog stuff tends to be extra sensitive....

I would think you would have brown out issues and problems with the servo due to an undersize ps rather than TC issues.
Did you check the wires inside the TC plug? They come apart, check the wires to be sure they are tight and not twisted so they can short out.