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Thermocouple Basics

A primer on thermocouples and mini-connector installation

Linsey Fan avatar
Written by Linsey Fan
Updated over a week ago

Simply put, a thermocouple is a temperature sensor commonly used in an array of industrial applications.  In conjunction with the right hardware/software, it's possible to reliably measure a broad range of temperatures over time - ideal for recording roast curves and optimizing profiles.

RoastLog can be used with thermocouples as well as resistance temperature detectors (commonly referred to as RTDs) when paired with the right data logging hardware.

  • thermocouples - compatible with RoastLog Data Bridge or Phidgets devices

  • RTDs - compatible with Phidgets devices

Read on to learn more about thermocouples.  For some additional details on using RTDs, check out this article on Using RTD temperature sensors.

“What type of thermocouple do I need?"

The short-answer is, "It depends." The slightly longer answer requires consideration of your current situation.

These days, it's common that new roasters come directly from the factory with temperature sensors pre-installed.  These will often be connected to PLCs (programmable logic controllers) that may automatically control functionality on the roaster, send the temp info to digital displays, or control safety gas shutoff valves among other things.  On the other hand, some roasters - more vintage ones, for example - may not have any temp sensors all.

The bottom line is...

  • If you have an existing thermocouple (or TC), determining what type is currently installed on your roaster and whether it is compatible are key steps.

  • For those who do not have an existing thermocouple, we can help you decide what size, length, and type of thermocouple is needed.  Note that “type” has a special meaning in this context.

Thermocouples “types”

Thermocouples can be made of different metals, which designates the "type". Two of the most popular are type-J and type-K. The current generation RoastLog Data Bridge is compatible with ungrounded type-J and -K thermocouples.

Regardless of the type, the TC must be ungrounded.

A few other general notes:

  • The only difference between TC “types” is the type of wire from which they’re made.

  • TCs are not interchangeable. This means that a device made for type-K thermocouples will not work with type-J TCs, and vice versa.

  •  If you have one TC, you cannot connect it to two readouts. Thermocouples cannot be “split” as you would splice speaker wire, for example.

How can you tell which type you have? Thermocouples are color coded, so you can look at the color of your wires or connectors and figure it out. The Omega chart of thermocouple color codes is quite handy. If you can’t see the individual wires, look at any of the connectors. Black means it's a Type-J and yellow is a Type-J.


Every thermocouple has two ends, so to speak.  The 'business' end sits inside the roaster and is where roasting temps are measured.  A stainless steel sheath, or probe, protects the junction where the two lead wires meet.  The opposite end of your thermocouple assembly connects to hardware (RoastLog's Data Bridge or a Phidgets device) which interprets the temperature measurement data.  This data logger literally sits between your roaster and computer or tablet.

Type-K thermocouple terminated with mini-connector (top);
Dual type-J/J thermocouple - one lead with bare wires, other terminated with mini-connector (bottom)

If you plan to use the RoastLog Data Bridge, you're thermocouple lead wires should be terminated with a mini-connector.  We designed it this way because it makes for a sure-fire way of plugging into the data bridge while respecting the polarity of the wires.

Here's a brief description of how to install a mini-connector on the end of your thermocouple lead wire. 

  1. Remove approximately 1/4" of the insulation from each of the TC leads to expose bare wire

  2. IMPORTANT: Visually inspect the leads to ensure the metal strands of each wire remains separated

  3. Removing the outer sheathing around the wires and gently pulling the wires away from each other will also ensure the two wires remain isolated

  4. Identify the polarity of each wire

  5. Open up the mini-connector housing by removing the screws - most easily performed with jewelers screw driver

  6. Identify the positive and negative screws associated with each of the connector prongs

  7. Wrap each wire around the respective screw terminal ('+ to +' and '- to -'), ensuring metal-to-metal contact, and tighten securely

  8. REMINDER: Make sure the lead wires are well separated to avoid creating a secondary junction. This will result in incorrect temperature readings!!

  9. Replace the cover on the mini-connector housing and use retention hardware (if included with your connector)

Type-J lead wire polarity (white = positive, red = negative)

Type-K lead wire polarity (yellow = positive, red = negative)

A brief comment on over-temp TCs

At a minimum, there should be one thermocouple installed in any modern roaster that is connected to some over-temperature circuitry. This is as a safety measure designed to shut off the gas supply if the roaster temperature gets too high.

Warning: For safety reasons, you should never disconnect your over-temp thermocouple or any other components in that system.

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