Stumbled upon this topic when I was searching for ways of preheating the Bullet using bean temp instead of IBTS.
I have given much more thought on how the Bullet preheats, especially after learning more on Scott Roa’s roaster warm-up techniques and his “between batch protocals” recently. I’m new to coffee roasting for sure, but I’m experienced in cooking and have learned a lot on modern cooking science prior to roasting coffee. The decision of trying to reach a target IBTS temp during preheat seems really odd to me now, it’s like designing a cruise control system to maintain a stable fuel output to the engine (by surfing pump voltage) instead of trying to maintain a stable speed of the car, let me elaborate below.
A drum roaster (gas or Bullet) transfers heat to the beans in majority by i) conduction (heated drum surface), and ii) convection (hot air inside drum). We know for gas roasters the conduction to convection ratio is about 30:70, the Bullet uses conduction a little more due to its way of heating (IH vs gas) and its smaller thermal mass comparing to larger roasters. For the conduction part of heating you may think you want a stable drum surface temp, however the drum surface temp changes rapidly on the bullet during roasting, especially when the drum of the Bullet is actually its heat source and is able to increase / decrease temp in a much more responsive and nimble fashion comparing to the drum of a gas roaster.
In a classic gas drum roaster, what you want to achieve during preheat is to reach the same thermal mass with the same amount of energy stored in the roaster system every single time you roast, to achieve consistency. The thermal mass of the system includes not only the drum, but also the roaster, the roasting room, etc. Preheating fully ensures the effect of uncontrollable factors such as ambient temp and humidity to matter less to the roast.
In the Bullet during preheat, once the drum surface temp (IBTS readings) has stabilised, it’ll wait for the bean temp’s ROR to drop to a certain value (not 0, but count as stabilised by the Bullet) before letting the user know it’s time to charge. The problem is that preheating takes time, soaking the machine and equlising with the surrounding temp takes time. Stabalising the drum (also act as the heating element) surface temp to the preheat temp only, and tries to use this drum temp to heat up the entire roasting system seems very indirect. Especially the final equilising phase, when the roaster system is approaching equilibrium (decreasing the gap to the target drum temp), the rate of heating is going to slow down, and the equilibrium is never / very slowly reached. This is evident with users reporting never ending slowly creeping in increase of bean probe temp after the Bullet has reported charging is ready. In addition, with the same target drum temp and different ambient conditions (mostly temp I guess) will result in a different amount of energy stored in the roasting system.
Using the analogy of cruise control, it’s like with a constant fuel output, you’ll get different speed on different road condition, faster on concrete pavings comparing to rocky bricks road. The quantity of fuel input is put to constant (+ve energy) while the losing of energy due to ambient condition varies…
This analogy can also demostrate what I mean by indirect of reaching equilibrium with a constant drum temp, at first the voltage of the fuel pump (= power in Bullet) increases to counter the startup resistance (initial friction of the moving pump, loss in heat energy, etc) of the pump (= heating the drum in the Bullet), once the constant fuel output is reached (= target drum temp is reached), the output of fuel is maintained (= maintaining drum temp). On the other hand, the car starts from 0 km/hr (= cold Bullet), the pump may have reached it’s targeted flow in 1 sec while the speed of the car is maybe 10 km/hr (= slightly warm Bullet). At this stage, the fuel output is kept constant, the acceleration of the car slowly decreases while its speed is approaching the theoretical speed of “fuel output = loss of energy”, where the equilibrium of speed is achieved (= equalised thermal mass of the roasting system). Sure, given enough time this constant speed could be reached, however wouldn’t it be more effective to try to actively maintain a constant speed rather than trying to reach a constant speed using a targeted enery output? i.e. accelerates to 80 km/hr before trying to keep a constant speed. In the bullet fashion would be to reaching a targeted bean probe temp and trying to maintain that in an extended preheat to make sure the roaster system has achieved the same preheat even if the ambient conditions are not kept the same.
Sorry for the really long post, I hope this could clearly descibe my thinking on this, as English is not my mother tongue. For now, I’m trying to use the roasting mode to quickly surf to my intended bean probe temp and try using the preheat mode (targeted IBTS) to maintain the extended warm up. However, the bean probe temp drifts a little when I change to preheat mode, and I need to find the corresponding IBTS target temp for that day (due to different ambient conditions). What are your thoughts?