Oh, a mini UDS. Yeah, at 25 CFM you have 5 times the CFM you need for a mini UDS. This will definitely lead to wild temp fluctuations like shows on your graph.
I don't think you will be where you can test your software like you want till you have a smaller fan. But here is a suggestion to make the best of what you have on-hand.
- Cut the intake of the fan to 1/8" of what it currently is. Tape over the air inlet is an easy way to do this.
- Set the target temperature up high, like 325F or 350F. This will require more air so your PID loop has a better chance of stabilizing that oversized fan at a reasonable speed.
- Use on/off control instead of just variable speed. Maybe that is what you are doing, not sure. There may not be a maintainable fan speed slow enough to keep the pit at the desired temp (meaning at some point you will lower the voltage down to the point where the fan either stalls, or doesn't have enought juice to start spinning the fan.
- Put a long pipe between the fan and the smoker inlet. This will increase drag which increases back pressure reducing the air flow rate at the same fan speed (typically). Might help, might not.
OK Joe Vertical Smoker, UDS, Komado Acorn, Weber Kettle Ranch, Weber Kettle, Stok Kettle