NOTE: If anyone reading this post is inspired to try something similar, you MUST be aware of potential hazards associated with DIY modifications that use 110VAC (household AC) and temperature controllers that could cause a fire hazard. This hazard could be the result of your system or temp controller being improperly configured, improperly wired or used in an inappropriate manner. What I describe below is for educational purposes only, and anyone building something based on this post is fully responsible for ensuring that it meets local electrical codes, and being aware of its potential for electrocution or fire. Also, see the last paragraph in this post for more details on potential failure modes.
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So we had this highly-rated slow cooker that we were using for making CBD-infused coconut oil for salves. Besides CBD, cannabis plants contain a number of other beneficial compounds called Terpenes. They are somewhat volatile so it is important to keep the processing temperature as low as possible. Some terpenes you might recognize are turpentine (not so pleasant for topical use), lemon oil, orange oil and a number of similar terpenes that find use in cooking or medical applications
Unfortunately, the slow cooker's temperature controller went crazy and would heat the contents of the slow cooker to the boiling point regardless of the temperature setting. This particular cooker had 4 settings -- Keep Warm, Low, Medium and High. It became no good on any of them. So what to do? Well, I so happened to have an STC-1000 temperature controller on hand. I originally got it to make a temperature controlled cabinet for making my own cured sausages and meats. It turns out that our basement is the right temperature anyway so the controller wasn't needed.
I took the slow cooker apart and found the heater connections, then cobbled something together to see how the controller would work to maintain the right temperature. At first, I went with a fancy spring-loaded temperature sensor that pressed against the bottom of the slow cooker crock, but the thermal path from the heater to the sensor was too good, so the contents of the crock didn't get anywhere close to the set point. I found it was necessary to immerse the sensor in the crock itself. For making CBD extract (or sous vide) this isn't a problem, because in both cases the "product" is in a jar or bag so the water in the crock doesn't come in contact with the stuff we're cooking up.
The in-the-crock temperature sensor worked just fine so I proceeded to make a more robust setup. I bought a plastic enclosure and a pair of male/female DB9 connectors, then machined holes in the enclosure to accommodate the controller and connector. I also had to do some creative machining (and carving) for the power cord's strain relief. Unfortunately, the strain relief wasn't quite the right size for the power cord so it wasn't properly captured by the strain relief. To address this, I did two things: I squirted a good dollop of silicon caulk into the strain relief to glue the cord in place, and I also put a tie-wrap around the cord on the inside of the enclosure, right next to the strain relief.
Today I had an opportunity to check the whole thing out. As in situations where 110VAC is present, there's a chance that sparks and smoke (or flames!) could come out of the box, but all went well. Here's a photo of the setup working to maintain about 160F in the crock:
One thing I didn't realize about the STC-1000 is that it ONLY supports degrees Centigrade, not Fahrenheit so now it is a METRIC slow cooker :)
CAVEATS
The STC-1000 will go into an error state and shut down if it loses connection to its temp sensor so that is one failsafe. However, if the sensor isn't put into the crock, or if it inadvertently comes out of the crock, the controller won't know that and will keep on heating until the water boils. This isn't too bad (yet): but once the water boils away it will continue to heat up and could become a fire hazard. Therefore, as it's currently configured it is NOT ready for unattended use. The controller itself could also fail so that's a potential issue as well. To address the over-temp problem I will buy a thermal-cutout switch that will be wired in series with the heater (and installed into the slow cooker) so it will interrupt the power if the temperature rises too much. This kind of secondary fault protection based on a mechanical thermostat (instead of electronic) is commonly found in laboratory ovens, and I thoroughly approve of this kind of protection. Don't make or use one of these modified slow cookers unless you include a thermal cutout switch!!!
