Preheat on Bean Temp Probe on V2


#1

Is it possible to do the preheat on the Bean Temp Probe and not IBTS? I find the IBTS stats static but the BT keeps rising?


#2

During preheat, the IBTS initially overshoots the preheat setting by 5C, then falls back and eventually stabilizes. The BT does the same, but with a time lag. For a 220C preheat setting, here is what I observed for BT:

preheat_temps

Like you mention, there is a small BT RoR of 2.4, so one might think that given enough time BT and IBTS would eventually equilibrate. But I don’t think they ever become equal.

After this first roast dropped at 218C, I started back-to-back for a second roast with the same 220C preheat. Here is what happened:

bac_temps

Based on this I would have needed to preheat for roughly another 10 minutes to reach the same BT as in the second back-to-back roast.


#3

My initial preheat to 230C generally takes 24+ minutes. Since I roast indoors at generally the same ambient temp, if I get “charge” any sooner than 22 minutes (happens only occasionally), I’ll cycle the PRS button to get more preheat time. I haven’t tracked the exact relationship of BT to IBTS but noticed the lag you mentioned.

On back-to-back roasts, the preheat depends on how long of a delay before I start my second roast while handling the first roast but usually is 8 to 10 minutes.


#4

I have discovered that I can trigger an error during PH if I do back to back (to back) roasts because the BT rises too high. It freaked me out until I figured out what was going on. Once it hits 247 C (or so) it rightly thinks the bean temperature is way too hot. (It would probably be flame on if there were actually beans in there.)

I guess my point is that the BT will continue to rise while it is preheating and cannot be counted on for an accurate charge temperature…


#5

There’s no mechanical connection between the Bean Temp probe and the drum. The only way the BT probe can sense the temperature changes is thru radiation and convection from the drum surface to the front plate where the BT probe is mounted.

Thermocouples are a very inexpensive, reliable and repeatable temperature sensor. But they have to be in contact with what they are measuring, i.e. the junction has to be in contact with the sheath of the probe and the sheath has to be in contact with whatever it’s measuring.

In the Bullet, relying on BT for measuring preheat temp isn’t a promising method since there’s no contact between the BT probe and the drum. All that the BT probe can tell us is the sum of temperature of the air around the probe and whatever is conducted into the probe sheath from the front plate. Hence the long slow creep up (initial preheat) or down (preheating for back-to-back roasts).

Bruce