I'm converting an kitchen top oven into a reflow oven and all seems fine except for the temperature measurements.
Using the same K-type thermocouple, but an external cold junction, the measurements are correct. Both the K-type's that have the 'cold junction' in the ovens control compartment, which heats up by design (for the bi-metalic temperature controller), are off by about 30 or even 40 degrees C.
Now I think that the cold-junction reference of the MAX 6675 and the actual cold junction temperature value don't match, as I think that the chip heats up much slower than the exposed metal of the thermocouple connectors. I've soldered one of the thermocouples directly to the PCB as the screw terminals broke, so heat transfer should be much better, which didn't make any difference unfortunately.
I'm using a predesigned PCB that features the MAX 6675, looks quite similair to this one: https://www.newegg.com/p/295-0026-001J6
I've covered the inside of the oven with heat reflective tape, which seems to work fine, but the control compartment still reaches temperatures of 40C when the temperature inside the oven is around 140C.
So how should I deal with this issue?
- Add heatsinks on top of the MAX6675's to allow for better heat transfer into the chip?
- Place one or multiple fans inside the control compartment to pull cold air through it?
- Use soft fluffy isolation material that they use inside house roofs?
- Preheat the control compartment to a fixed temperature of 40/50/60C?
- Using thermally conductive epoxy? (I think it's better to attach the metals directly to the PCB instead of running them through a connector first)
EDIT: I'm testing the oven with the MAX6675 chips out side of the hot compartment and the temperature is still way off. The only difference is that the probe connected to my DMM, has about 8 or 9cm of wire inside the actual oven and the probes that I installed inside the oven, only have the hot junction inside the oven. Could this be the explanation of the big temperature difference?
EDIT 2: I've wrapped the thermocouples wiring with heat reflective tape and sticked them right above the surface of a metal bin. The temperature readings make much more sense now, I'll try a PCB later on to see if it still comes out black.