I ran a few test roasts over the weekend. All a older Myanmar bean. The tests were with three different airflows.
The lowest read 0.03"wc on the Magnehelic and I consider the lowest air and still pull the flame exhaust out. The second is a more normal airflow of 0.05"wc, which I've typically used on many quality roasts. Then, the final roast was with 0.08"wc. With this higher airflow full gas had trouble maintaining the RoR during ramp. My max fan is still considerably higher than at 0.15"wc, but this is way too high to roast with.
All roasts I charged 32oz at 370F BT. But notice the first (0.05" baseline drum temp was hotter since outside drum temps were cooler).
The bean was a rather old Myanmar bean I got for nothing. The slow air and fast air roasts, I smoothly adjusted gas such that the profile for all was matched (best I could do). Beware, my MET (inlet air) is not reliable and must pick up a lot of hot/cool air turbulent. My BT Phidget has an issue with being steppy and so my RoR is a bit noisy.
The three roasts matched pretty well. All ended within a 15 second window and within 2 degrees. My ET and RoR closely matched as well.
Slow Air roast
Fast Air roast
I still need to cup and pull some espressos with these roasts. But what I really wanted to see is contained in the plots.
My gas markers show gauge pressure multiplied by 10. So P10=1.0KPa, P20=2.0KPa and so forth. The air markers are similar but inches wc x 1000. So F30 = 0.03"wc.
My conclusions are that I cannot change drum temperatures very much with airflow for a set profile. Other observations are that my inlet air temperature (MET) with low airflow hit record high numbers even though gas used throughout roast was lowest. Highest airflow did not reduce MET all that much relative to baseline. Also interesting is my ET (exhaust temp) seems to want to match between roasts in order to have matching BT (regardless of airflow). The gas required for fast air roast was max and I had to completely shut off the gas with low air roast during development.
I'm not sure what all this means other than I need to find another way to reduce my drum temp (air and gas don't work). I do have a few more knobs. Namely my drum RPM (which was set to 63rpm) can be bumped to 72 or dropped to 50. Then, I can change my charge weight. Would a half load act similarly? I'm wishing my roaster had an adjustable burner height.
Al, I think your low air roast must be lower airflow than the one I did with 0.03"wc. I did not change my air during roasts, so maybe I too would see jumps in ET.