Archive for April, 2013

Cooking with Science!

Posted in Electronics, Things with tags , , , on April 5, 2013 by Spark

When I heard about sous-vide cooking, I knew I had to make a one.  It’s a way of producing perfectly cooked meat, and it involves messing around with electronics.  Right up my alley!   I decided to put one together for my girlfriend as a Christmas present.  She loves cooking, and making the project as a gift gave me a deadline and ensured I’d actually finish it.

Sous-vide cooking involves vacuum sealing food (usually meat) and cooking it in a temperature controlled water bath.  The idea is to set the water temperature to exactly what you want the internal temperature of your food to cook to, then leave it for a few hours.  It won’t overcook or dry out.  Then you can take it out, sear the outside to make it crispy and brown, and have a thick, juicy steak that’s cooked perfectly evenly medium rare straight through.

I decided to make a temperature controller that would switch power to an off-the-shelf crockpot.  I bought a thermocouple and a PID temperature controller with an autotune function and rigged up a proof of concept with a rice cooker from a thrift store.  I configured the PID controller to use the built-in relay and spliced it into the power cord of the rice cooker, and dangled the thermocouple into the water in the rice cooker.  It took a long time to get it running.  The thermocouple had three leads, two blue and one red.  I couldn’t find any description of which lead was which.  Most thermocouples only have two leads.  I eventually figured out that the two leads that were the same color should go to the + and – pins, and the odd lead should go to the R pin, but the setup still wouldn’t work.  I eventually replaced the thermocouple and it worked fine – the first one must have been either damaged in shipping or possibly fried when I hooked a multimeter up to try to determine which lead was which.  I never did figure out what the thermocouple labels on the PID controller meant, or why there was a third lead.  If anyone has an idea, let me know!

It took a while to figure out how to properly wire the thermocouple

Once I figured out the thermocouple setup, I bought some pre-sealed steak from Meat House in Arlington and tried out my bare-bones sous vide cooker.  I sat my rice cooker on a counter with the temperature controller spliced into its power cord and the thermocouple hanging into it.  I ran the auto tune function on the PID controller, set it 130 degrees F, and let it come up to temperature.  I threw in the sealed steak and left it for a couple hours, then pulled it out and seared it on the grill.  My setup may have looked like a couple components and a bunch of wires strewn across the counter, but it produced a damn good steak!

I found the clicking of the built-in relay on the PID controller to be pretty distracting, so I ordered a solid state relay to use on the final version.

I found a single power outlet with a built in switch at Home Depot that could provide both the outlet that the crockpot/rice cooker would plug into, and the power switch for the sous vide cooker.

Power switch and output outlet

Here is the wired up sous vide cooker with all the parts attached to the enclosure.  You can see the back of the output outlet/power switch, the solid state relay sitting on top of a large heat sink, and the PID controller.

All wired up!

This is what the finished sous vide cooker looked like from the front.  Pretty slick!

Aww yissss

Parts list:

PID Temperature Controller          $32.50
Solid State Relay                                  $8.50
Heat sink for relay                               $4.25
PT100 thermocouple                         $22.50
Switch and Power outlet combo    $7.99
Strain Reliefs
Enclosure

Additional Resources:

Guide to cooking with sous-vide
Cooking time and temperature reference

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