Temperature regulated stir plate

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Argyll Gargoyle

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I bought a stir plate with a heater, since I build my starters in the basement. Sometimes it gets too cold, and because I step up from slants, I don’t want the whole process to take forever.
Well, it turns out the existing temp adjustment is pretty worthless. It goes from 0 to 11 with hardly any adjustment in between — and inconsistent to boot. I cooked my last starter, and that tears it!
So I ripped out the guts and implemented a PID controller using a small SSR, a digital temp sensor, and an arduino board.

4D288110-8FFD-4D19-B1D3-1DA62C9A559D.jpeg


The temp sensor is the small black cord on the left-hand edge of the hot plate. It goes into a hole drilled into the aluminum plate. Control wires for the SSR and motor are run out of the box for testing, but will be put inside along with the arduino. The unconnected pcb near the arduino is the old control board.
Here it is settling a step in temp from 30C to 35C. Time samples are about 2-3 per second

A44FD18D-7B63-4683-91B9-480EB836A8C8.png

I can provide additional details if folks are interested. I haven’t seen many builds like this (or any?)
The stir plate was 50-60 bucks, SSR board $10 or so
 
Pretty cool idea! Does the temp sensor have to be submerged in the starter?
No - the temp sensor is intended to be permanently mounted to the hot plate surface. It turns out that the closer you can place that sensor to the heating element, the better in terms of a stable and well-controlled temperature. The most common issue that folks tend to see (in keezers for example) is a huge overshoot when stepping the set point. This usually happens when using a thermowell sensor because there’s a big lag between when the heater turns on and when the beer at the thermowell comes up to temperature. Folks usually have better luck just taping the sensor to the outside of the carboy. Well, same story here—putting the sensor close to the heater helps improve the speed of the response and reduce overshoot.
There may be a temperature offset between the starter and what the controller believes the temperature to be, but this should be a fixed number that doesn’t change much. I plan to use my handheld temp sensor to check what temp the actual starter is, then adjust the setpoint accordingly.
 
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