As a prelude, I'd like to offer that I have obviously read the famous question here: Sensor for ultralow (to -85°C) freezer monitor
I have set up a K-Type probe from Adafruit and their MAX 31855K breakout board with SPI. I can read the data all right, and get consistent results at room temperature.
What is problematic is that when I put this sensor in a -80°C freezer, the probe seems to bottom out at ~-65°C, whereas the actual temperature probe of the freezer shows -80°C.
I though my computations were false (the 14 bit temperature is tricky to bit-shift and 2's complement convert), but I have made the math myself by printing the bits and using paper and I find consistent results with my code.
So yeah.. do you guys know if this is a non-linearity of the sensor at these temperatures?
Is the K-Type thermocouple not adapted? (I thought it could go down to -200°C) Could this be a problem on the MAX 31855K side?
Any hint at what could be wrong?
I'm planning to check different places in the freezer (right now the probe is on the top compartment, I'll try the bottom tomorrow), and another freezer too to eliminate any "freezer-specific" influence, but if there is something I overlooked, can you help me?