I am a relatively new Bullet (V2) owner, having bought mine in May 2022. I have completed 20 roasts (plus 3 seasoning roasts).
Most of my roasts have been 500 g batches, other than the “establishing” roasts I do with each bean variety to establish what the FC and SC temps (IBTS) are, which are 250 g.
I roast using the methods espoused by Morten Munchow which means roasting to color, for which temp is a proxy and varying the power and fan to govern development times. My general goals are about 9 mins to FC and 4 mins to end temp.
I generally end the roasts around 215°C to 217°C, well short of SC. I take decaf a bit darker, perhaps 220°C. I have found the IBTS to be very consistent across batch sizes with the FC temps generally being within 1°C.
I roasted my first 1 KG batch yesterday and things behaved quite differently. I missed most of my milestones, which didn’t surprise me, but what I found confusing is that well before FC, the bean probe temp exceeded the IBTS temp. I’ve never seen this in my previous roasts and am unsure how it is even possible for the bean probe temp to indicate greater than the IBTS temp.
This left me unsure when to drop the beans. When I finally ended the roast, the bean probe temp was 223.5°C and the IBTS was 215.2°C. The roaster was producing considerably more smoke than it usually does (managed to set off the smoke alarms in the house!). The beans were maybe half a shade darker than I was shooting for. It is still quite drinkable - am drinking a cup, now., but I would like some technical insight into why this happened.
Gray, I’d suggest using the search function and search “roasting 1kg” or “1kg roasts” or “IBTS and BT cross” or variations thereof. I know there is at least 1 or 2 threads already about this topic.
Yes scaling up from 500g to 1kg is very different. I did that. Yes the BT and IBTS lines are going to track very close just before FC and even cross. You’ll see that in my 1kg profiles on RW as well. My theory for this is because you have a lot more bean mass that takes up that air space in the drum therefore the BT and IBTS is going to be closer in their readings. Now if the lines cross drastically it might be time to do a deep clean.
I looked at that 1 kg roast. Odds are good you could probably benefit from cleaning the IR sensor lens (the clear glass of the sensor- there’s no refraction) when B-Temp & IBTS are reversing position. Also 1Cs seems a little low- I look for that to happen at ~400°F though that can vary with bean variety (vs. being purely roaster instrumentation).
The recommended approach is to use a Q-Tip + alcohol. Alcohol doesn’t dissolve the roasting ‘stuff’ that gets on that surface blocking some of the IR, but it softens it. And till something better comes along it’s the recommended approach. First clean the outer ‘rubber’ area as well as the inside of the view port (clean Q-Tip each pass- don’t reuse them). With the view port clean, do the same with the IR sensor. I do 2 or 3 passes on the IR sensor, clean Q-Tip each time. Cleaning the IR sensor is not at all satisfying- I have yet to see anything on the Q-Tip. Yet there is a difference in the IR reading of the IBTS.
So, how often? I check maximum temp difference between B-Temp and IBTS at IBTS about 260°F That’s where I see the greatest difference between the 2 values for my 550 gm batch size; it’s a little hard to see initially because there are artifacts in the data which modulate the IBTS reading- just look for the greatest value. For me, that difference will be about 44-46F° for a clean sensor; I let that drift down to about 41F° before I clean the IR sensor again. For smaller batch sizes that max difference will be larger than the roughly 45F° I get; for larger batch sizes the difference will be smaller.
Don’t try to do this during a roast- too much going on. Just make note of the difference when reviewing your roasts later… just make it a routine part of reviewing profiles post-roast.