Differences between temp readings

I am completely new to the roasting world and super excited to learn more. So bear with me as this may come off as a silly question!
One of my biggest struggles I’m having right now is deciphering between the bean temp and IBTS temp. The way I have been reading and recording things is based mainly off of the bean temp. Is that correct? Any explanations of the differences in temps and what/how I should be relating the two would be so greatly appreciated! Any and all tips/tricks are welcome!
Thanks in advance!

Fellow novice 20 roasts in: I recall reading several previous threads on IBTS v Bean temp probe readings. You are probably better off to do a search for previous mentions and they may answer your questions.
I am using IBTS as main guide/info source so far, but recall interesting discussion about initial startup temps being much wider difference and hence causing reliability concern.


Hi! :smiley:

Here is how I understand it:

The BT probe is a metal thermocouple that is partially buried in the beans. This is the kind of probe used on roasters for many years. It reacts to changes in temperature slowly. The beans heat the BT by physical contact. If there are very few beans (like 150 grams) the BT will mostly be measuring air. If the drum is full (800g+) the BT will have much more contact with the beans. The drum speed also changes how much contact the BT has with beans. For this reason I almost always roast same weight and drum speed.

The IBTS is an infrared sensor that measures the surface temperature of the drum and beans. It is in the top of the faceplate, looking down towards the back of the drum. When the drum is empty, IBTS is measuring drum temperature. When there are beans, IBTS measures a combination of bean surface and drum temperature. IBTS responds to temperature changes immediately.

In a Bullet, the coils heat the drum, the drum heats the beans and air, and the exhaust heats the rest of the machine. High fan brings more cool air into the drum, which cools the drum and beans.

Because the drum is usually hotter than the beans, the IBTS usually reads higher than BT. If the IBTS is dirty, it will give a false (low) reading.

The preheat step uses IBTS. If your IBTS is dirty, the actual preheat temperature will be too high. A good clue it’s time to clean IBTS is when the temperature lines converge:

The fan does not run during preheat, so there is little exhaust. I think this explains why the first roast of the day is always a little different. The exhaust heats parts of the machine the IBTS cannot see. So IBTS says the Bullet is fully warm but there are areas outside the drum that are a little cooler.

This is everything I think I know about Bullet temperature and heating. Experts, please correct my mistakes and add whatever you can!


Great overview!

Thank you so much for that great advice! I defiantly ran with it and it provided me with a much better idea of what I am looking at! :slight_smile:

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