BBQ guru logic question

Scotty, I'm not 100% sure I understand the behavior of your unit as you describe it. I'm not a BBQ Guru employee, nor do I have any specific knowledge of the alorithm that they use in their units, but I do have nearly three decades of experience working with industrial temperature control, so here's how I think the Guru folks do it:

Virtually all temperature controllers today use, at their core, an algorithm called PID control. This stands for Proportional, Integral, Derivative control and represents the three ways the controller responds to error, defined as a deviation of the measured temperature from the setpoint temperature. In practice, most use only the "P" and "I" portions of the algorithm since the "D" can cause instability.

"Proportional" control is just what it sounds like. The unit's output is increased proportionally for every degree under the setpoint the Egg is running and decreased by the same amount for every degree over.

"Integral" is a little more complicated, but intuitive if I explain it right. This function keeps adding/subtracting more output the longer an Egg remains off-target. For example, if you've set the Egg to run at 250* and it's running at 240, the Guru will run the fan at the output determined by the Proportional algoritm above, but it will also keep looking at the error and will continue to increase the output using the Integral until the Egg hits setpoint. At that point, the Guru's Proportional algorithm will contribute zero to the outout, with the unit's set to whatever the Integral has ramped itself up to. In fact, integral is often called "reset" because of precisely this action.

Controllers use a 0-100% output to manipulate whatever their final control element may be - in the case of the Guru, that's a blower to stoke the lump. Since the blowers controlled by the Guru aren't variable speed, they instead vary the amount of time the blower is running out of a pre-determined period (I think they use 10 seconds), chopped up into 10 segments. That means if the controller's output is calculated to be around 40%, the Guru will turn the fan on for 4 of the 10 available seconds.

Now to your problem....

I think what is happening is that your unit, when starting, may have some leftover Integral output left in it, hence the blower running at times even though the Egg is over temperature. Eventually, though, the Integral realizes it's way over where it needs to be, and drops to zero. At that point your fan turns off for good, trying to cool the Egg. Eventually, and I've had this happen to me on low-temp cooks, too, the fire dies out.

I've gotten around the issue by starting my Guru out at a higher temperature, say 200*, then walking it back slowly a few degrees at a time by just gradually lowering the dome setpoint temp. The Egg manages to cool in a short enough period and the Guru begins cycling the fan again, at which point I know the fire's still okay, and I ease the setpoint down again. I've cooked as low at 150* using this method, and I probably could go lower if I wanted.

Sorry for the book.

Go to the guru web site and question them direct. They are real good about answering.