August 8, 2021

Hacking the Fresh Roast SR800 home coffee roaster

Todays topic is thermocouples.

The SR800 has a temperature sensor buried inside. It is not a thermocouple - it is almost certainly an RTD. It measures the hot air being fed to the roasint chamber, so it will be significantly above the bean temperature. This is certainly useful, and worlds better than no sensor at all, but we can do better.

Adding thermocouples to the SR800

There are at least 2 reasons to add thermocouples to the SR800. One is to get one right into the middle of the bean mass. The other is to be able to feed temperature readings to some kind of software for display, logging, and perhaps control.

Off the shelf solutions

The usual way to do this is to buy what you need from an outfit called Phidget. They used to sell something called a 1048, which was a nice gadget with USB on one side and thermcouple connections on the other, but of course they don't make this any more. Now you need to buy two boxes and a cable to join them to get the same functionality. One box is what they call a "hub", which has USB on one side and connections to go to various other products they make on the other. Then you buy a quad thermocouple box to go with that (and the cable to connect the two) and away you go.

These items cost $35 and $30 respectively. Add a pair of type K thermocouples which you can buy from Phidget as well, and that is $16 each, so you are looking at a total of $97.

This is a fine solution if you aren't an electronics hacker like I am and has the added advantage that it is immediately supported by the Artisan software you almost certainly want to use.

The following video and links should give you more information, as well as take you to where you can buy this stuff. Note that the video ends with some discussion about "what would I do differently" and one thing he say is that he might use just one thermocouple. He notes that the "B" (bean) and "E" (environment) thermocouple track each other closely with a fairly constant offset between them.

Less expensive solutions

Less expensive as far as money, but much more in terms of time. But playing with things like this is what I like to do. You can get type K thermocouples on Amazon for less than $10: I bought one of these and have now let it sit around gathering dust for over a year (since August, 2021). A big advantage with it is that it is a stiff rod about 8 inches long and can be immediately installed into the SR800 once I drill a hole for it. The downside may be its own thermal mass which may make it sluggish responding to temperature changes. We will have to see.

Once you have a thermocouple, you will need some electronics to get data from it:

I ordered two of the MAX31850 boards and another much smaller K thermocouple from Adafruit.

If we do just one thermocouple, going with the Amazon device and a single MAX31850 breakout, we are looking at $22 instead of $97. We will still need some kind of microcontroller and software. I have a drawer full of microcontrollers and small single board computers, so that extra hardware is already sitting here looking for a purpose. Writing software is what I do anyhow. Artisan is open source, so in theory I can do whatever it takes to get my data into Artisan as this project unfolds.

So, I ordered one of the cheap thermocouples from Amazon for $7, along with two of the MAX31850 breakouts and a stainless tipped thermocouple from Adafruit. Adafruit offers $5.10 USPS shipping, so my order is $47 dollars with tax included. I'll note that I am definitely not going to be saving money compared with buying the Phidget stuff, but I'll have more fun. At least I think it will be fun.

DS18B20 temperature sensors

I bought a 5 pack of these from Amazon: These are good for temperatures from -55 to +125C, so they are by no means suitable for any aspect of coffee roasting. But I mention them here since they are kicking around in the same box with my thermocouples. Note that the K thermocouple is good for up to 400C (750C), so it is what you want.

Have any comments? Questions? Drop me a line!

Tom's coffee pages / tom@mmto.org