I am working on writing a control program for my pellet smoker. I am having an issue with the meat thermometer that came with the smoker.
I have a MAXIM 31865 ADC that is connected to a Raspberry Pi. Attached to the ADC I have a thermistor that is used as a meat thermometer. This thermistor (unknown type/manufacturer) is inside of a metal sheath and has 3 wires that were previously connected to the smoker. There are 2 internal wires that connect to the thermistor and then the metal cable sheath itself was also connected to the original control board on the smoker.
I only have the 2 internal wires that actually connect to the thermistor connected to my ADC. The third wire (thermistor sheath/cable sheath) is not connected to anything. If the thermometer is sitting on the grill or on my desk I am able to get an accurate measurement from the ADC. However, if I pick up the thermistor or touch it more than just lightly brushing it I instantly get an overvoltage/undervoltage error on the MAX31865 ADC that doesn't go away until I put down the probe.
I assume I need to ground that third wire by clipping it to a metal surface on the grill or back through the power supply, but I am trying to understand what is happening here. I tried measuring for a voltage drop/increase while touching the thermometer and I couldn't see anything happening when measuring with a multimeter, and I don't understand why it would be a problem anyway.
(I am not an electrical engineer, just a hobbyist.)
Image of how it is wired I put together before I started. The black triangles go to a strip of copper on the solderable breadboard. The empty triangles go to a different strip of copper, and then the grounds are merged to a single ground pin on the Raspberry Pi. The small black boxes are ferrite beads. I pretty much just followed exactly what the Max31865 docs said, but since I had a single power source I split it with a ferrite bead between the source and the analog voltage input.