The fear of stalling during first crack vs. speeding into second. Ive been trying to start into first at about a 8-12degrees/min. rise rate then cut a little heat about 2/3 way through 1st so at end of first is about 5-6degrees /min. but no less.
Peter In my last post the temp. rise I was referring to were thermocouple bean temps. As your post shows monitoring bean temp is key to a great roast profile. What are you roasting with now? Didn't you build a TO/ with a BM drive? I'm now working on a 2-8# version with propane CO. my first model BM/TO does up to 2# .http://coffee-roasting.blogspot.com/