Preheating Temperature inconsistent

Hi, everyone!
Does anyone notice on their bullet that their BT probe reads different preheat temps with every batch? Sometimes with a very big difference in temp?

For example I would set preheat at 527F and the charge light flashes when BT probe is at 320F
Next time I do the same preheat, it would flash charge at 375F or 380F.

I get that first batches will mean the entire system is cooler meaning a lower temp on BT probe but didn’t expect it to be so big of a difference.

Do you guys wait until BT probe temp is within a certain range of the IBTS temp for preheat?
I wanted to do that but sometimes it takes quite a while for the BT probe to hold temp

Yes. That’s the nature of the beast. The front plate where the BT probe is mounted gets heat during Preheat indirectly (radiation & convection) and a tiny amount directly (conducted) thru the 6 mounting screws attach via threaded inserts embedded in the insulated shell of the roaster, and thru the drum shaft & front plate-mounted ball bearing assembly. You don’t have the addition of heat by conduction from the beans till after you charge the drum.

On your 2nd (and later) batches the front plate and the BT probe have to cool before they reach the Preheat target for the drum. The trick is to try to hit the same Charge temp on #2 (& later) roasts. One Bullet owner here (@quartzglen ) recommended the first batch be a throwaway- old, dead beans which he used just to stabilize the internal temps of the roaster. Odds are good most of us aren’t willing to part with Inventory just to heat-stabilize the roaster, but if the first batch is crappy I guess that’s what we’re doing anyway.

I try to get around the throw-away approach by

  • doing a long Preheat at a higher target temp than I will use for #2 (and up), then
  • when I see BT hit the same temp I hope to see for each later roast I launch a Recipe (which is set to the lower Preheat temp), then charge the drum.

I tried making 2 versions of the Recipe with the only difference being the Preheat setting but ended up shooting myself in the foot by not getting the 2nd Recipe (with the planned lower PH temp) started before getting BACk-to-back PH started with the lower PH temp setting… too many things to keep track of for my aging brain.

The Bullet is tightly scripted to operate a certain way both manually & with Playback & Recipe controls- ‘close enough’ doesn’t cut it. I’ve bumped into that wall a few times and settled on this workaround. Later roasts (#3 and up) are no problem. Just be warned YMMV.


Edit- years ago I saw an ad for ceramic beans that were intended to be used in place of a throw-away roast. I was afraid of damaging the seasoning coating of the drum or getting chips of ceramic in the coffee so I never experimented with them.

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I’d often used preheat temp of 240c/464f, and I always wait until BT reached or above 167c/332f to charge.

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Thanks for the detail reply!
For some of my roasts I see that the BT temperature was actually way higher than the first roast meaning I had to wait for BT temp to drop down which will take a long time.
It just seems odd to me that sometimes the 2nd batch preheat BT temp is much higher, sometimes it is lower.
I will just try to cool down the machine by opening the hatch or set it in cooldown mode if BT probe gets too high.
If the BT probe is too low I guess I’ll just charge at a higher IBTS temp
Either way it’s kind of a hassle and wastes a lot of time =/

Sometimes I run a blank roast for first roast of the day. Like, 3 minutes or less, low power, no beans.

My theory is the exhaust fan helps warm up the rest of the machine. Still experimenting.

I wonder why the fan is not used during preheat. Seems to me half the machine is cold and stealing heat from the drum.

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Hi Wuhc55,
I guess the main reason is because the waiting on the first preheat is not long enough to reach or equal to the 2nd or 3rd back-to-back roast…
So the way I set my “predict goal of Preheat BT temp” is to actually using the past experience (assuming the room temp does not change too much) of the 2nd and 3rd B-T-B roast preheat temp when Bullet calls for the “charge”
In another word, the 167c BT preheat temp that I am using is mostly happened with the following B-T-B roasting. That way, you will be able to avoid cooling down experience.