I've connected a max31855 with a type K thermocouple connected to it, to an atmega328, which I'm powering via a USB port on outlet like this one:
All works fine until I stick the metal end of the thermocouple probe like this one:
into the soil (that I'm measuring temperature of), at which time I get unpredictable readings. If I wrap duct tape around the probe to insulate it from the soil, again all is fine. At first I thought maybe just noise from the switching regulator, so put a clamp-on bead on the USB cable, but that didn't help.
I'm guessing some kind of ground loop through the switching regulator for the USB port and back to house mains, but not sure how to fix it, while still using this port, which is very convenient for this project. Any ideas?