Two temperature measurements are essential for controlling and monitoring your roasts in directly heated drum roasters. One probe measures the temperature inside the bean pile (bean temperature, or BT) and the other probe measures the temperature of the hot gases blowing past the beans (environmental temperature, or ET).
In an air roaster, it’s common to measure the temperature of the air entering the drum from the burner (inlet temperature, or IT). The inlet temperature is the most effective way to control the roasting process in this type of machine.
Probe Type and Thickness
The most common type of temperature probe installed in roasters is called a thermocouple — this is the same device used in instant-read thermometers. Various types of thermocouples are available, made from different materials; the two types most suitable for coffee roasting are called K-type and J-type. You can read more about the different types of thermocouples and how they work in our علوم التحميص (قيد الترجمة) course.
Thermocouples may be either grounded or ungrounded: grounded probes respond to temperature changes faster than ungrounded ones, but they can give inaccurate readings in certain circumstances. We therefore recommend using ungrounded probes.
A thermocouple measures the temperature of the tip of the probe itself — it doesn’t measure the temperature of the surroundings directly. For example, the temperature reading from the BT probe, with its tip positioned within the bean pile, doesn’t correspond exactly to the temperature of the beans. Instead, it depends on the temperature of the air between the beans, as well as that of the bean surfaces.
The probe itself takes time to heat up or cool down, and this causes a delayed response to temperature changes. A thinner thermocouple takes less time to heat up, so it responds more quickly to temperature changes. A very thin thermocouple can be fragile and easily broken, however. A good compromise is a probe of approximately 3 millimetres in diameter.
Best practices for thermocouples:
• J- or K-type
• Ungrounded
• Approximately 3 mm in diameter
Some roasting machines employ other types of temperature probes rather than thermocouples to record certain measurements.