PID Tuning, Lid Detect, and Charcoal Consumtion Rate

[Dave Pasma]

Hello All. Please take a look at my UDS brisket cook chart and initial config screenshot from this weekend. I configured it to run the fan (stock HM blower) along with the damper.

The oscillations went away after I dropped the fan max to 75%. Should I have set the max fan to 75%? I suppose that a higher max fan % has the same affect as a higher D parameter? Or, do they fight each other - causing the oscillations?

Lid detect triggered when I put the brisket on, it did not trigger again. 6% means it should trigger at 235.

I used up all of my charcoal this time - which has not happened before. 3/4 of a basket lasted 9 hours. I usually have a good amount left after a long cook. Possible factors: 1) I ran at 250. Usually I go low and slow at 225. 2) running the fan with the damper? Does this make the cooker less efficient? I know a few of you have said that fans can have a cooling effect. Could it be that there is more air flow through the cooker? If the fan PUSHES cooler outside air, that will make the fire burn hotter. If there was no fan, the coals draw the air in.

 

[Andy Snider]

Thermodynamics is a *****.
I suspect the reason you got smoother response after reducing your max fan speed is that @100%, you are pushing too much cool air into the camber, lowering the temp, causing the HM to call for MORE air, until the fire catches up. Then it gets real hot real fast, overshooting, so the HM closes the damper, choking the fire & cooling it off real fast. Somewhere along the line, your fire change & needed more air/fuel than you had -ash? The hot/fast -> choke could have caused a bit of an inefficient burn as well, or it could be variation in charcoal. who knows - you can't test w/o burning it, you might as well have meat above it when you 'test' it.
Your PID settings aren't the issue. Your biggest swing was ~30* over ~15 minutes. Your cow didn't even notice, based on the meat probe.
The Pit Temp, which is what the HM reacts to, can be wildly out of sync with what fire is doing, which is what the overall system will react to. Lid open, too much air, adding 15lb of cold meat, etc will 'confuse' the HM & take a while to iron out.
You could tune your airflow, which if you look at the graph, 75% wasn't far off to hold temp - fire was already acting goofy. Or add thermal mass to keep the system temp more stable while the HM figures out how to react.