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Thermocouple installation and placement
Thermocouple installation and placement
Linsey Fan avatar
Written by Linsey Fan
Updated over a week ago

This article will explain in plain terms your options for installing thermocouples on your roaster.  (If you're installing RTDs, the same basic principles apply.)

Getting your thermocouple situation sorted out is often the most confusing part for those unfamiliar with these little devices. For the uninitiated, this can feel daunting and overwhelming. In reality, there are only a handful of details to be concerned about when getting setup. For now, let's focus on finding a home for your new friend(s).

Getting Started

Let's get started by figuring out your particular situation... Every new RoastLog user will fall into one of two categories:

  • You have an existing thermocouple and it’s connected to a device which displays the temperature reading

  • You do not have an existing thermocouple

Before we move forward, some may find it helpful to take a moment to review our article on Thermocouple Basics - especially if you are retrofitting previously installed TCs.  If you want to jump into the installation, read on!

Installing dedicated RoastLog thermocouples

Nowadays, most modern roasters come with TCs pre-installed.  However, setting up new TCs, specifically dedicated to recording roast profiles, is arguably the easiest and least complicated approach to start using RoastLog. While this may sound like a big undertaking, in reality it’s very straight-forward.

Whether these will be the first TCs installed on your roaster or if you plan to install new TCs alongside existing ones, the process is the same:

  • Identifying the correct location

  • Drill a hole

  • Tap threads into the hole

  • Install compression fitting

  • Adjust thermocouple insertion depth

Installation location

Invariably, the question of where to install a new thermocouple has to be addressed. There isn't really one definitive answer for this since roasters can vary drastically between different makes and models. However, there is a general location which you should aim for regardless of the roaster. This can be best shown in a photo:

General location for a thermocouple installation. Photo of a 2013 Probat P12.

In this photo, we can see that the drum rotates clockwise and creates a bean curtain approximated by the green shaded area. To get a good measurement of what the beans are experiencing, the thermocouple should be placed well within the bean mass. It should be obvious from the photo that if the thermocouple was installed higher up on the face of the roaster, there is a chance the TC would not be immersed in the bean mass while roasting smaller batch sizes.

In this case, the TC was installed directly in the door and is merely one of many options. When determining the appropriate location for your probe, take into consideration the location of the curtain, drum rotation direction and location of the fins inside the drum.

It’s also important to be aware of the drum fins; making sure to find a spot where you can insert the TC 2-3” into the drum without hitting any fins while the drum is rotating.

Drilling a hole

Once you’ve identified the location, it’s time to drill the hole. This hole will ultimately be tapped to accept the compression fitting which will hold the thermocouple in place. As such, you need the correct drill bit and tap. We happen to keep a small stock of these on-hand as well.

A compression fitting, 1/8”-27 NPT tap and matching drill bit.

Beyond the items above you’ll need a good drill, some muscle and a tap handle. Drilling can require healthy doses of patience, especially when drilling through steel.

We can supply all of these tools and parts, if needed. You'll need to provide the rest.

Tapping threads

Once the hole has been drilled, it’s time to tap some threads. Resources on how to tap threads abound on the Internet. So, we’ll refer you there for details. A couple of notes we will give:

  • Be patient. Forcing a tap can break it and leave it stuck in the roaster.

  • Use a liberal amount of cutting fluid

  • Use a tap bit handle rather than a wrench

Install the compression fitting and adjust TC depth

A thermocouple probe with its compression fitting. The threads on the right will be turned into the threads you’re cutting.

Once the threads have been tapped, clean out the hole and install the compression fitting. Once that is in, slide the thermocouple in and tighten the outer fitting (left side of photo) which will hold the thermocouple in place.

Replacing an existing thermocouple

Most modern roasters come with at least one thermocouple which is connected to some type of digital readout/display. If you’re in this situation, you may want to consider replacing your existing TC with a dual-element thermocouple.  This allows the existing hole to be re-used without needing to drill and tap in a new location.

Side profile of dual-element thermocouple - two TCs are housed within the stainless steel housing. In this diagram, one is Type-J and the other is Type-K.

Dual thermocouples consist of a single probe sheath containing two thermocouples. The TCs within the probe can be type-K/K, -J/J or -J/K.  RoastLog works with either type-J or -K, so simply choose the 2nd TC within the pair to meet the requirements of your device.

Take note what type of TC is currently plugged into your existing reader; and be sure to replace it with the same TC type.

A dual-element type J/K thermocouple. Note that there is only one stainless steel probe. 

In the photo above, the Type-K connector is a yellow dual-pronged mini-connector (right side). In contrast, the Type-J thermocouple terminates in bare wires (red and white).  The bare wires make it possible to connect directly to a digital controller/readout via screw terminals. It's common to see these connected to a Watlow digital readout or something similar.

Whether it's bare wires or a mini-connector, the thermocouple will always be connected to some data logging/display device.  For details on adding a mini-connector to bare thermocouple wires, check out our Thermocouple Basics article.

Where should we get our thermocouples?

A quick online search for TC suppliers will yield an endless number of vendors. Omega is a company known to supply high quality thermocouples, for example.  As a convenience, we also stock a limited selection of dual-element J/K and single-element J and K thermocouples.  If you have an existing TC and wish to replace it, we recommend the dual-element J/K.

Note: The TCs we keep on hand are generally 1/8" or 3/16” diameter by 4” long with 6’ of stainless steel over-braided wire. If you need different dimensions, we may be able to help you get what you need.

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