Out of curiosity, I put a k type thermocouple in the bean chute by the exhaust channel and used an Amprobe recording thermometer to record the exhaust temperature (named ExT) as it left the drum during a roast. The Amprobe can record and output a CSV file. I used the RW CSV download and superimposed the exhaust curve on the roast curve in a spreadshee. Image attached (hopefully). Maybe a bit crude, but some interresting results. I am presuming that cold air leakage from the front plate/drum gap isn’t affecting things too much. If I use the IBTS trace as actual bean temperature, then the ExT (after 2 minutes) is below the actual bean temperature. This tells me the fan setting (F3) is mostly causing cooling of the beans. The little blips in the ExT trace are from the tryer. Since it’s a pretty direct path from the tryer to the exhaust, I doubt it affects the general in-drum temperature much. I noticed that changes in power did not affect the curve much. Of course this behavior is probably highly dependent on batch size and ambient temperature. I’m curious what you all see in this and how you interpret it. Jim
With induction heating “drum temp” is a bit funny, because the drum is the heat source.
You could suck out all of the heat in the drum, and still have a hot drum.
So “drum temp” language might need some extra nuance when discussing the Bullet?
@gabyritaseek.qiAO mentioned a quote from Scott Rao that the Bullet is the least convective roaster he had ever used. And the aillio engineers claim the reason they did not include an air temp sensor is because they do not consider the airflow important enough, because the Aillio is a conductive roaster…. Which it may be just about the only conductive roaster, since most are roasting primarily with convective energy and conductive is secondary. Even Probat engineers have been noted as stating their machines are doing more convective than conductive heat transfer.
In personal use, I find I wish I had the heat build of F1, and the airflow of F5 or higher.
Adding heat to the airflow system helped, but there is too much plastic and the IBGT1 sensor all dislike this idea very very very much. 
ymmv.
And thank you for posting this graph, I had never seen this visually, but rather just take notes, so this is fascinating and I am learning so much!
This graph mirrors my experience. While I have measure different airflow locations and gotten different temps, so I do think the air temperature is likely higher inside the drum than what we measure, at least the measurements are consistent. And they do suggest with near certainty that the airflow is not actually getting to roasting temp targets, or that there are air leaks in the system at that point.
Seeing your airflow curve is a lot nicer looking than mine is.
What is your batch size and airflow there?
I notice that I can be flat lining and baking with my airflow and flicking and burning with my IBTS. It does translate into bland an uneven and weak tastings roasts. Like one would expect.
That roast was 450g. The fan was at F3 almost the whole roast until I bumped it to F4 near 10:30 minutes to purge some chaff and to control the profile so the RoR didn’t rebound. I usually jump to F5 now at 1C start plus 90 seconds. The ambient temperature was about 65F with 25% humidity. I live at 6000F altitude, so that has some affect too.
I stopped recording the air temperature on a regular basis because it is a lot of work to make the graphs and it is pretty much the same story roast after roast.
I didn’t try using F2 through most of a roast and that might show a little improvement. The ambient temperature and humidity does somewhat affect the initial time that convective heating occurs, but not really that much. I find I prefer roasting at cooler ambient temperatures. Mine vary from 63F to about 80F winter to summer.
I’m sure you’re right about the air temperature in the drum varying. Especially with low air flows I’m sure the temperature is stratified with the hottest at the top. Given that the air flow design pulls air out near the top of the drum, I have to wonder if the temperatures I measure by putting a thermocouple in the chute plug are perhaps skewed higher than the general temperature around the beans. Given that cool air comes in the back, there is probably a temperature gradient back to front too. Higher drum speeds probably mix the air better and would mean more consistency.
I think you’re right that the airflow helps keep the roast more even. Any hotspots on the beans from contact with the drum are tempered by the cooler airflow. A high drum speed probably helps there too.
I would like to have more convective heating too, but I don’t think the design permits that. I don’t think preheating the input air would make that much difference and (as Aillio pointed out) this would compromise the cooling dynamics of the roaster components. Actually, I see the cooling effect by a faster air flow to be a useful tool for a between power number heat adjustment.
BTW. I discovered that the inside design of the load port plug fits very nicely over the tryer port. So I got a spare and every time I pull the tryer, I just cover the hole with the plug. I think this keeps the airflow in the drum from stalling a bit when air is sucked in through the tryer hole. It may be trivial, but it makes me feel I’m being more consistent.
Jim
Makes sense to plug the tryer hole. I stopped using the tryer, but it really helps being able to smell the roast process.
I’m thinking the machine hates heat because of the plastics and the electronic components. So any benefits of warmer environment are offset with things like reduced power. But this is my recent experience.