Saturday, October 5, 2013

Sous Vide Cooker

Sous Vide Cooker

I built a sous vide cooker. For those of you unfamiliar with the process, sous vide means under vacuum and refers to the fact that before being cooked the food is vacuum packed. This is typically done with a device like a Seal-a-Meal or Foodsaver. The food is then cooked in a water bath to a precise temperature. This method of cooking has some advantages. It can prepare food to the same degree of "doneness" throughout. So, for instance, if you like your beef medium-rare you can cook a roast so that the whole piece is medium-rare, from the outside to the middle, from the thick end to the thin end. Also, you're able to control the temperature precisely so it will be medium-rare, or whatever you like, every time. Further, the food can be prepared in advance, then held for hours with no chance of over cooking.

As you can imagine, however, one must be careful with this type of cooking so that the bacteria in the food are killed. For that reason precise temperature control is essential. To achieve this a PID (proportional-integral-derivative) controller is used. This is essentially a very clever thermostat. Rather than taking into consideration only the current temperature versus the set temperature as a conventional thermostat does, a PID controller manages how quickly the device responds to an under or over temperature condition, the undershoot or overshoot of the temperature, and the temperature oscillation.

There was an article in Make Magazine Volume 25 by Scott Heimendinger that laid out in detail the design and construction of a homemade sous vide cooker. I showed it to my brother John who is something of a boffin in the area of food preparation. He had bought a sous vide sometime back and was familiar with their operation. We looked over the article and he commented that it seemed like it would make sense to build one from a cooler to better stabilize the temperature and to reduce energy consumption. So that's what I did.

Below is a picture of my creation showing a project box fastened to the top of the cooler. It contains the PID Controller, the control panel of which can be seen, a solid state relay and heat sink for switching on and off the heating elements, and various electrical connections.

With the top open it is clear how much I borrowed from Scott Heimendinger's design.

The viewer can see the three heating elements, and the black box is a submersible pump to circulate the water during cooking. Between the heaters and the pump the thermocouple that measures the temperature of the water is visible.


Potential Improvements

While I'm very happy with my cooker's performance there are a couple of things I would change in version 2.0. I have read that these heating elements are extremely sensitive to being powered when not immersed in water, and can burn out almost instantly. Thus I would put an interlock switch such that power would be disconnected if the top is opened. I might also replace the coffee cup type heaters with a heating element from a small hot water heater. I would mount this through a hole below the water line. This, of course, presents the problem of making the penetration leak-proof. This would be complicated by the fact that plastic coolers have double walls, both of which are flexible.

I might also use a slightly larger cooler. The size hasn't been an issue but cooking for a very large party might be problematic.

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