The bean probe does not only measure the temperature of the beans. It is affected by the metal carcass where it is fixed – and by the air passing by.
The bean probe is quicker affected by the airflow than the beans. Metal absorb heat quicker than the cellulose beans. Think of sitting on a bench in the park on a cold day; the difference of a wood bench and a metal bench. Coffee beans are more like wood than metal.
Here I have tested the bean probes sensitivity to airflow.
This test is on my Bullet R1 roaster (a test roast with old beans, not meant to taste good). The bean probe is a NTC precision type.
Look at the graph after 8 minutes:
In the last 4 minutes of the roast I jumped between a high airflow; F7 and a low; F2 (and once F1). Each time approximatly 30 seconds stretch. I chose F7 because already at F5 the airflow starts to cool rather than increase heat transfer. That is with 400 grams batches – at larger batches it takes higher fansetting.
High airflow made the ROR go negative within 10 seconds.
Low airflow made it go positive within 10 seconds.
During this shifting airflow the Bean temperature (BT) did anyway rise from 187 to 191°C, measured at low airflow. Probably due to heat transfer from the drum, where the heat setting was constant at P3 during these 4 minutes.
Clearly the bean probe is not only affected by the actual bean temperature – but also of the airflow. As we long expected. And as Rob Hoos also wrote in his Roast Magazine article this month (the sep/oct 2017 issue). He also talks about it in this video https://www.youtube.com/watch?v=IQjxCHE10HQ
Read more about the Bean Temperature under Roasting Basics.
Note: The first 8 minutes of the roast was because I had just changed the bean probe. At first the test was only to see the difference to the old one. But during the roast I got the idea to test the airflow impact on the BT reading.