I recently decided to try a new roasting method too I built a SC/TO for about $75. I've been experimenting with it for a few days now and I'm very satisfied with the results. The SC/TO gives me a lot more control than my Behmor. I'll keep an eye out for an old bread machine next time I'm at a yard sale or thirty shop so I can give this method a try.
No doubt a heat gun/bread machine has all the right stuff for roasting coffee. Your design has the top enclosed to reduce environment temperature gradient. I would add a window and light so you can monitor the beans during the roast. Now add an environment temp probe just below the heat gun and just above the bean mass. This is the roast temp you throttle control to achieve your desired profile. Completely pre-heat the vessel before adding the beans.
Given only one temp probe, yeah, I would likely put it in the bean mass every time. A reasonable proxy for environment temp may be obtained as you have for the preheat; Without a bean load, the BT becomes the ET probe. You know where to set for a steady 350F in the chamber, so then find what setting will give you the required finish ET (maybe in 450-475F range). Drying phase likely will work around 350F, then ramp to the finish temp setting for 3-4 minutes and you should be close; hold that setting until completion. My poppery is not quite so predictable in this regard, and may be mostly due to variance in air flow restriction of the bean mass. The heat gun should be more consistent in this regard, allowing for ambient temperature changes.
Strictly speaking, the bean mass should not be allowed to loose heat to any part of it's environment through the entire roast; not until it is dumped for cooling. Any measure you take to improve your roaster in this regard is effort well spent.
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