A problem I’ve had with my chaff collector hood is that it retains too much heat thus allowing the beans to heat up faster than they ought to. When I set the PID to 400 and let ‘er rip I get roasts that look like this:
Notice the mottled color, uneven roasting, and the fissures. The coffee is bitter with no depth.
Edit: I’ve recently learned that the effects of over-heating the beans is called “tipping”, because the small ends of the bean tend to get burned first.
Tonight I did a batch following a more gentle curve and got a much better-looking roast:
They already smell better than the bad roast, and they’re 10 minutes old. I managed to keep the temperature right inline with this recommended roast profile by setting each minute’s taget temperature at the start of the previous minute. For example if at 4 minutes I was suppoesd to be at 180 I would start adjusting my PID at 2:50 which takes about 10 seconds to increase by 20 degrees. Then over the next minute the PID will gradually bring it up to the correct temp. They key here is that when the PID is close to its target temperature it won’t go full power. If you’re 300 degrees away from your target it’s going to apply heat 100% of the time which apparently leads to crappy roasts like you see here.