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.