What is ET (Environmental Temperature) in coffee roasting?

ET stands for Environmental Temperature - the temperature of the air inside the roasting drum, measured by a probe positioned in the drum environment rather than in contact with the bean mass. It's one of the two primary temperature readings displayed in most roasting software alongside BT (Bean Temperature).

While BT tracks the heat absorbed by the beans themselves, ET reflects the temperature of the thermal environment surrounding them - essentially how hot the air and drum atmosphere are at any given moment. ET typically runs significantly hotter than BT throughout a roast, as it measures the driving heat rather than the result of heat transfer into the beans.

The relationship between ET and BT is informative: a large gap between ET and BT in the early roast indicates significant heat input relative to bean absorption; as the roast progresses and beans heat up, the gap narrows. Some roasters use ET trends alongside BT and RoR to understand the thermal dynamics of a particular roast. Roasting software like Cropster and Artisan plot ET as a separate line on the roast curve. On machines like the Aillio Bullet, the exhaust temperature (ExT) is also logged separately, giving a third temperature reference point alongside drum BT and drum ET.