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Goal: Understand what thermocouples are, which type you need, and how to connect them to a data logger for use with RoastLogger.
Prerequisites: None — this article is useful for anyone setting up temperature sensing for the first time.
A thermocouple is a temperature sensor commonly used in industrial applications. In conjunction with a compatible data logger and RoastLogger, it's possible to reliably measure temperatures over time — ideal for recording roast curves and optimizing profiles.
RoastLog supports both thermocouples and resistance temperature detectors (RTDs), each requiring a compatible data logger:
Thermocouples — compatible with Phidgets VINT devices (recommended) or the legacy RoastLog Data Bridge
RTDs — compatible with the Phidgets RTD Phidget (TMP1200)
For more on using RTDs, see our Using RTD temperature sensors article.
"What type of thermocouple do I need?"
The short answer is: it depends on your current situation.
These days it's common for new roasters to come from the factory with temperature sensors pre-installed, often connected to PLCs or digital displays. Older roasters may have no sensors at all. The bottom line:
If you have an existing thermocouple, the key steps are determining which type is installed and whether it's compatible.
If you don't have a thermocouple, we can help you decide what size, length, and type you need.
Thermocouple "types"
Thermocouples are made from different metals, which determines their "type." The two most common are type-J and type-K. Both are compatible with Phidgets VINT devices.
Regardless of the type, the TC must be ungrounded. TCs are not interchangeable between types — a device configured for type-K will not work with type-J, and vice versa.
Thermocouples are color coded, making it easy to identify the type. The Omega Engineering thermocouple guide is a great reference. In general: black connectors indicate Type-J, and yellow connectors indicate Type-K.
Connecting Your Thermocouple to a Data Logger
Phidgets VINT devices (recommended)
The 4x Thermocouple Phidget (TMP1101) uses screw terminal connections — simply insert the bare wire ends and tighten. No special connectors required. Phidgets also sells an assortment of thermocouples directly if you need to source one.
Legacy RoastLog Data Bridge
If you're using the legacy RoastLog Data Bridge, thermocouple lead wires must be terminated with a mini-connector. Here's how to install one:
Remove approximately 1/4" of insulation from each TC lead to expose bare wire
Visually inspect to ensure wire strands remain separated
Identify the polarity of each wire
Open the mini-connector housing by removing the screws
Identify the positive and negative screws for each connector prong
Wrap each wire around the respective screw terminal (+ to +, − to −) and tighten securely
Keep lead wires well separated to avoid a secondary junction — this would cause incorrect temperature readings
Replace the cover on the mini-connector housing
Sourcing Thermocouples
We no longer stock thermocouple hardware. Thermocouples are widely available from manufacturers and suppliers — Phidgets stocks an assortment of compatible TCs and RTDs, and Omega Engineering is another well-known supplier. If you have questions about which thermocouple to order, reach out to us at support@roastlog.com.
Safety: Over-temperature Thermocouples
Any modern roaster should have at least one thermocouple connected to over-temperature safety circuitry, designed to shut off the gas supply if temperatures get too high.
Warning: Never disconnect your over-temp thermocouple or any other components in that safety system.