An accurate temperature controller for low temperature long duration (sous vide) cooking.
I started by looking at Seattle Food Geek's Immersion Circulator and the core of the design is largely based off of his plans.
The key difference was that I wanted more flexibility so instead of wiring the relay straight to some immersion heaters, I wired it to an outlet that the heating device can plug in to. This allows me to use a rice cooker which is more energy efficient and has a more stable temperature than a plastic tub.
The rice cooker tends to heat fairly evenly so there was no need for a water pump or air bubbler to circulate the water, which also helps reduce the complexity of the project.
Minor modifications include using a C14 inlet for the power input, adding a fan for cooling the relay, and wiring the sensor to a 3.5mm jack for easy replacement.
Building the Controller
I started by ordering all the parts and once they arrived I began looking for a suitable enclosure. I settled on a 6x6x4 inch conduit box which had room enough for everything, but still wasn't too large.
After spending some time determining the arrangement of the parts, I used a Dremel to cut out the holes for all the parts and the fan. I chose to mount everything on the lid of the box for easy access; open the box and everything slides right out.
Once the parts were attached to the box they were wired together and then tested before the box was closed.
Tuning the Controller
Please read How PID Controllers Work for some background.
Once everything is assembled, the parameters of the PID controller need to be changed to correspond to your situation. Most PID controllers have an auto-tune function; however this does not always yield the desired results. Consequently I opted to tune the PID parameters manually.
I recommend reading the PID tuning guide for the Sous Vide Magic controller to learn how to tune your PID manually. At a high level the process is:
- Set P to 20, I to 0, D to 0, and find the optimum P value by halving the P value until the current temperature fluctuates around the target temperature.
- Next, set I to a large value (e.g. 600), reducing it if the change in temperature isn't corrected fast enough and increasing it if the temperature fluctuates wildly.
- Lastly, set D to be 25% or less of your I value. I used 10%.
So for my rice cooker I used P=10, I=300 and D=30. With these settings I found that the initial heating takes a while but the temperature is very stable once heated.
Using the Controller
Next turn on the controller and set the target temperature by pressing the set button and changing the temperature as appropriate with the up, down and left buttons. Press set again once you've selected the desired temperature. Now wait until the water warms up to your desired temperature and then add your food.
The traditional first dish for sous vide is an egg since they are simple, cheap, tasty, and have a shell which eliminates the need for a vacuum sealer. Cook the egg at 64.5° C for 45 minutes to get an egg with smooth whites, neither runny nor rubbery, and a thick, pudding-like yolk. Enjoy!