Preheat should use the Bean Probe rather than IBTS

Roasting on the Bullet R1
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I’m still pretty new to the bullet, only a few weeks of using it at this point, so I could very well be missing that there is a way to change the preheat procedure to use the bean probe but I’ve just missed it.

Anyways, I’ve been noticing major inconsistencies with the preheating depending on if it’s my first roast of the day or depending on how long I let the roaster soak. These inconsistencies are fairly obvious based on the bean probe temperature, which seems more sensitive to soaking temperatures.

Firstly, I noticed that when the roaster tells me to charge, the bean probe temperature is not stabilized. Given a couple more minutes it may rise around another 20-30F. Either that probe is especially slow, or has a thick shell, or something. Either way, it seems more consistent if I wait until the probe comes up and stabilizes.

I just had a roast with a Nicaraguan coffee that was my second roast of the day, back to back. I set my charge temperature to about 20F than my previous roast of a Kenyan coffee, but I noticed the bean temperature was about 50F hotter than what it was when I charged the Kenyan coffee. This coffee ended up tipping and I think it may be that despite just a few more degrees from the IBTS, the bean temperature was way higher.

Anyways, it seems like the bean probe temperature is more reliable and stable. Maybe I’m just suffering from learning too many things at once and it’s doesn’t actually make a difference. Let me know what you think.

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The Preheat routine in f/w adjusts and maintains drum temp. It uses IBTS and Power exclusively to do that. The bean probe, prior to charging, measures the air temp inside the drum (poorly!) and the temperature of the face plate (again, not very well); the probe is basically useless for measuring radiated heat in the ir spectrum. Without beans to contact the probe during preheat, those are the only 2 sources of heat for the probe to measure.

The concept of any drum roaster is to put heat into the drum and from the drum into the beans. Once the beans start heating, the probe conducts some of that heat to the sensor inside the probe. It’s at that point the BT probe starts to register the temp of the beans- anything before that is outside the purpose though it does offer some insight into thermal stabilization prior to charging. Any heating of the probe prior to charging the drum is incidental. That said…

If you’re getting tipping you are using a preheat that’s too high for the beans & batch size being roasted. Drum speed is involved too. For a completely different approach to preheating and roasting intended to avoid that very thing search here for @quartzglen and ‘heat soak’… near the end of 2019.

Bruce

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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?

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In addition, I don’t find using the IBTS temp for dropping beans very reliable. I find the IBTS temp seems to be affected by the drum surface temp, however I’ve only been using small batches of 325g for now. To counter the exothermic effects of coffee beans after first crack, one would decrease the power to avoid the flick at the end if following Rao’s approach. I find my IBTS temp actually decreases towards the end while the BT probe still increasing, which I find similarities with ET probes dropping towards the end on classic gas drum roasters as well.

I think what happened is that one actually stalls the power of the roaster at the end so much that the air temp inside is actually decreasing while the bean is heated mostly due to the exothermic first crack of themselves.

As I said, I’m a beginner in roasting, I’m not trying to give a view or something. It seems just like observations that worth of discussion with the unique IBTS system on the Bullet.

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The IBTS temperature during preheat is an average of the whole drum, including shaft and vanes. As the majority of the energy is stored in the drum, it makes most sense to use the drum temp as a preheat indication, and not the bean probe. The bean probe is very sensitive during preheating especially across different bullets as a small gap between drum and front plate will create a draft that will change the temperature.
Our findings is that the best possible way to preheat is using the drum temperature to do so. And I agree that when the Bullet says charge it has not fully stabilized. This is however a compromise as most users don’t want to wait too long.
If consistency is very important to you, then you will know that the first batch will be a bit different (slower) than other batches, but this can be taken into account.
For your individual roaster, using the bean probe might be helpful as well, but you will not be able to compare this value during preheating to other Bullets,

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And as the Bullet uses induction heating instead of the conventional gas, I would expect the heat distribution on the drum to be more even than a hotspot at the bottom where the burners would traditionally focus.

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And bear in mind Scott Rao’s advice is largely geared to large volume gas powered roasters, which the Bullet is not. So not all the “advice” applies evenly. It’s a bit like trying to match driving advice for a 18 wheeler to a sports car. Still driving in principle yes, but some things don’t equate…

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Here is my way of best utilizing Bullet’s dual-temp sensor during preheat. First of all, batch size makes a difference and my bullet may not be same as yours. So the temp I am using may not the same as yours. The goal is to reach the most consistent roasting and not to wasting time at the first preheat.

  1. Using IBTS to set the Preheat temp, by design we are all the same using Bullet
  2. When IBTS reached the preheat temp and the Bullet believes it is stabilized(ready to charge)
  3. I will check the BT temp before it reached to a certain value before I do my actual charge.
  4. In my experience, If I set my preheat temp IBTS@250c, I will wait until the BT reached 170c~173c to do the actual charge (batch size is around 700g)
  5. The reason to choose BT@170c~173c is based on the data collected with back-to-back roasting. During B-2-B roasting, I’ve noticed that most of the time when my Bullet reached IBTS@250c, the BT is around 170c~173c. That way I don’t have to wait any time to do my next charge of the 700g batch.

Again there are lots of variables that mine is different than yours…batch size, preheat temp, room temp…and your own roasting profile…

In a way, I am using Bullet IBTS to take control of the preheat, then to use a consistent BT temp to decide when to charge. But you may need to experiment what BT temp you would like to use with some 300g batch size. The beauty of this powerful machine is if one understand how to best utilize the dual-sensor.

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