IBT - merging with Bean temp

Over the past 20 roasts my Drum Temp (IBT) and Bean Temp have been merging within a minute after first crack. This is new, it never used to do this always being 5-15degrees apart until I ended the roast. Any idea why this is happening now?

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Have you inspected the IBTS inside of the front of the roaster? Maybe there is dirt on it. Just a guess.

I have seen the same phenomena with all my roasts since implementing the IBTS. However, mostly seen with bigger batches as the BT then get closer to the IBTS.
Actually I often see FC happening when the two lines cross. So I am just guessing now but thinking that it has to do with the fact that the bean mass get exothermic just before FC - and that the IBTS actually see this as a loss of energy (lower surface infrared radiation) whereas the BT probe actually feel this additional surrounding heat and increase the reading?

For reference, this is what the curves used to look like…

This is what they currently are doing…

Another ??? observation. Compare the IBTS curve before YT. The curve is wavy on the converging (later) roast. I have started noticing this lately on my roasts but I do not have a converging late curve yet. It would be interesting to see if you looked over all of your roast curves to see if this was also part of the trend. Have you checked to see if the IBTS is clean?

I just noticed this waving on my roasts yesterday. I was worried something was going wrong. Glad to see others experiencing the same thing.

Please check your IBTS sensor and possibly clean it. Also if you can take a few pictures before and after cleaning we would very much like to see that.

The wavy early curve started after about 20 roasts and has continued ever since, I have noticed others with the IBTS have the same thing happening as well.

The converging/crossing curves tends to happen more with batches over 700gms, with bigger batches the lines actually cross with the IBTS dipping below the Bean temp by the end.

I just cleaned the IBTS and here is a before and after picture, will get to roasting another test batch in the coming days.

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Did you clean the shiny glass of the sensor with a cotton bud and a drop of alcohol?

Yes, the sensor was cleaned with cotton swab and drop of alcohol. I just completed four roasts since cleaning and the IBTS and BT seem to be back to normal with a 8-12 degree difference after first crack till end of roast. Compared my graphs back to my first 15-20 roasts with it and readings are consistent with those.


To continue with this, I have roasted approx 13kg since cleaning the IBTS and the BT and IBTS have started getting closer once again, also first crack is about 1 degree later then it was with the same profile after cleaning.

so what does it mean? no cleaning till?

13kg total and what batch sizes and trop temp ie 2nd crack reached?

I dont know what it means, just observations after having roasted 80kg total with the IBTS and having my data changing. Hoping Aillio will have some answers/recommendations for us soon as it feels we are in uncharted territory.

Why is the early curve so wavy?
How often and how should the IBTS be cleaned in order to maintain consistent roasts?

I’ve had to clean my IBTS every 11 kg or so.

wau 80 kg thats a lot do you use as a shop roaster