IBTS / Thermocouple delta

Hi Everyone!

Probably a pretty basic question but; how much of a difference should I expect to see between the IBTS and the thermocouple? I have seen anywhere from 90c at charge and around 20-25c at the finish of my roasts. I recently cleaned the IBTS sensor using alcohol and a q-tip but it did not change.


Cleaning the IR sensor of the IBTS is perhaps the most unsatisfying maintenance chore with a Bullet because you won’t find anything on the Q-Tip swab. Whatever is left behind that interferes with sensing IR radiation is probably white or at least doesn’t contrast significantly with the white cotton of the swab. On top of that alcohol does very little to dissolve roasting snot… takes a few repetitions and crossing of fingers but the results don’t show up till the next time you roast.

What follows is just my opinion… YMMV.

To know when I need to clean the IBTS I track the maximum difference between IBTS and BT at about 250°F/121°C during each roast. For my 550 gm roasts, that difference will be about 45F°/25C° for a clean IR sensor; when that difference drops to about 41F°/23C° it’s time to clean the sensor.

And the difference is specific to a 550 gm batch size. If you roast a smaller batch the difference will be larger and for a larger batch the difference will be smaller. In addition the temperature where you see that maximum difference will also differ depending on batch size. So it takes a little research for each user in order to accommodate the needs of their specific roast plan. IMO, relying on a calendar or number of roasts isn’t a dependable criteria as the contamination of the sensor can be affected by how dark you roast (!).

I use IBTS to determine drop temperature. The effect of a dirty sensor is that drop temperature gets higher as the sensor gets dirtier (so too does drum temperature during Preheat!). Since I have always relied upon a specific IBTS temperature to drop/eject the roast and that temperature gets higher and higher as the sensor gets dirtier, I felt I might be chasing the rabbit down the hole. I asked someone whose opinion I trust (@bradm ) for his thoughts and he suggested I consider relying on time from a specific point in the roast, say time from 1C, or maybe in my case I could use the time from 2C since I’ve been seeing differences in that time during a session and from roast-to-roast. I’ll be giving that a try next session (thanks Brad!!).

Btw, if you clean the sensor and you don’t see an increase in the max difference between IBTS vs. BT, you probably didn’t get the sensor clean.