Adding a Thermometer to a Coffee Roaster or Air Popper

A digital thermometer with a thermocouple probe is the best way to add temperature measurement to your coffee roaster. Here’s how!

Realizing that our old post on adding a thermometer was out-of-date (so old that I wouldn’t even follow my own instructions I had posted there), I thought it’s time to update the material. Here’s is what I actually would do to add temperature measurement to an air popper / coffee roaster.

Of course, as a dedicated maximalist (in all the wrong ways, usually) I couldn’t just make a simple “how-to DIY ” video. I had to complicate it. As far as the steps to add a thermocouple, there is not much to it. So you can skip to 8:20 in the video to just see the procedure. Yawn.

But before that I need to have a word about all the caveats and conundrums of temperature measurement in coffee roasting….

Wouldn’t it be great if you, me and everyone who roasts coffee, on a stovetop, in a popper, in a 120 Kg drum roaster, could speak the same language on coffee roasting temperatures?

Wouldn’t it be great if we could just say, “I roasted that coffee to 438 fahrenheit, and if you do too it will taste great like mine did.”

Alas, coffee is a cruel master. It won’t let us off that easy. In reality, few people are measuring the same thing when they are measuring temperature. And even when they are, the readings likely will not calibrate. Coffee is like society, diverse, complicated, and often a pain in the #[email protected]

But having some sense of temperature while roasting, when used in combination with our senses (sight, sound, smell) as we roast, further empowers us to make good roasting decisions. By “good” I mean this; repeatable roasting results, and generally roasting coffee we want to drink! Consistent readings from a thermocouple do not, by themselves, make for roasting success. But it’s still worth doing the modification to whatever machine (or non-machine) you roast on.

Adding a Thermometer to a Coffee Roaster or Air Popper

Links to Items in the Video: Coffee Roaster Thermometer and Thermocouple


So what are the basic steps to add a thermometer to a coffee roaster or air popcorn popper?

Here are some considerations, and steps to add temperature measurement:

  • You could simply drop the thermocouple into the top of the air roaster or popper, but it will move around in the air flow, and the spinning bean mass … so that will change your temperature readings. Plus it will be in your way.
  • Ideally you will be probing the coffee directly. If the probe isn’t touching the coffee from the start of the roast, ok. But hopefully it is contacting the bean mass at the more critical roast stages, nearing first crack and beyond.
  • If you are going to drill into any machine, as I do in the video, be sure you know that you won’t be near any electrical connections. Also you need to make sure you aren’t going to change the air flow, such as drilling through the fan housing … that’s a bad idea.

Steps to Install a Thermocouple in an Air Popper Coffee Roaster

  • Find out the safe route for your drill bit, measuring it inside the roast chamber at a place as low as possible, so it will touch the coffee when roasting. Transfer that point to the outside as your drill mark.
  • For the thermocouple I am using (50mm long, 3mm diameter), I chose a 1/8 inch drill bit to accommodate the 3 mm thermocouple probe.
  • Drill the 1/8 hole all the way through the outer housing and into the roast chamber.
  • Enlarge the hole in the outer plastic housing (and only in the outer housing!) with a 13/64 drill bit: My thermocouple as a threaded connector and lock nut on the shaft. Using the lock nut would be great but makes it difficult to reassemble the popper. I rely on the threads on the shaft, which are M6 (metric 6), and a 13/64 drill bit makes a hold that allows me to force those threads through the plastic housing in a way that secures the thermocouple firmly. If you have a Tap and Die set to cut M6 threads, well use it!
  • Plug the thermocouple into your digital thermometer. All done!
  • Remember, while your readings might not calibrate with everyone elses, they should be consistent relative to your own roasts. So if you have first crack at 375 f and mine measures 395 f, we just need to allow for that when comparing notes. As I point out in the video, each installation of a thermometer will be a different, reading a combination of bean contact temperature and the roast chamber environment. So variation is expected!

Image Gallery for the Thermometer Installation


FAQ Questions About Adding a Thermometer to an Air Popper Coffee Roaster … And About the Car !

The car?

It’s a 1965 Barracuda and it runs, but I can put my feet through the floor pans in the back (Yabba dabba do!) . Gotta fix that… (PS this is my storage space, we don’t store cars at Sweet Maria’s!)

Could I use this thermal probe with a Behmor Coffee Roaster?

That’s my next project! Although you can use the Behmor control panel to check the temperature of the onboard thermometers, I want to see if an additional probe would be helpful. But it couldn’t probe the bean mass as it roasts … at least I can’t see any way how. But if it is probing the roast environment, would it be consistent? Would it be helpful? I’ll try to find out, or find out if one of you already found out!

You say you like “raw wire thermocouples” but you are using a stainless steel one here – why?

I do like the naked thermocouples, the type not inside a metal “well”, because they are so responsive. I have tried a lot of ones that are encased, like this one, that are not very responsive for coffee roasting. With a faster roast, like in an air roaster, you want the readings to shift with the roast quickly because 10 seconds can make a difference at the end of the roast. I found that a very narrow probe (this one is 3 mm) is quite responsive though! And with an air roaster and rapidly spinning coffee, the naked probe will move around, which means it might not be measuring the same thing before and after it shifts position. But we use naked ones on our Probat 12 kg roaster and like them. Sweet Maria’s sells both types FYI

Is there any danger drilling into the popper / or other roasters

Yes, of course! As you can see in the video, I know what’s located in the spot I am drilling. If you don’t know, then don’t do it. Be smart about this.

Any other questions?

Use the comments section below and we can update this FAQ to answer your questions!

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