Cheap Sous Vide, Part 2

So, I’ve been contemplating this fine piece of gadgetry know as a thermal circulator. All the popular chefs are using them for sous vide. They, are, as I mentioned earlier, ludicrously expensive for an apparatus which may not be useful to me. The obvious answer is to see if one is available cheaper used. The cheapest used one I could find was about $600, which is still prohibitively expensive.

So, let’s use some of this high-powered engineering education and experience I’ve spent all these years acquiring. Our desired device has one singular function in life: maintain a water bath at a precise temperature. So, we need some way to heat the water and some way to control the heat. If we control the heat, we control the temperature.

So, how do the bright boys who make these things do it? A quick look at an existing model reveals an immersion heating element, some kind of control system, and a pump to smooth out any gradients. All right, that looks pretty straightforward.

Hmm. Let’s cruise some odd back corners of the Internet. A 2500 watt immersion heater element for $2.95! That just might work. If I do some back of the envelope calculations*, I figure I need about 200W to maintain a temperature of 140°F compensating for convective and radiative heat loss. I’m ignoring conduction since we shouldn’t be touching much with our pot. Hmm what do I get from a 277V element if I run it at 120V? Damn, now I have to remember my sparky equations, or at least look them up. So, scribbling furiously on the same envelope after consulting references, I get about 470 watts from my chosen element.** A little bit more math and the heater should get 10 liters of water hot enough in about 50 minutes. Screw that, I’m impatient. We’ll run two in parallel and heat the water up twice as fast. Oh, wait, how much current is that gonna draw, anyway? 8 amps? Fine, we can easily pull 8A on a household circuit.

Right. That’s the heating. Let’s see what is available for a control system. Now, those of you unacquainted with industrial process controls may be unaware of a device known as a temperature controller. It’s a pretty little pre-engineered box that requires a power source and a temperature sensor to work. You hook everything up and enter a desired temperature in your controller. The controller then checks the temperature sensor against your desired temperature and based on the result either switches something on, or switches something off. In our case, we want the switch to come on when the sensor temp reads low, and go off when the temperature sensor reads high. This is the default configuration for most controllers. So, where do we find a temperature controller? I started off at Omega, since they make everything under the sun for temperature measurement and control. But, as befits many industrial suppliers, they are expensive. So, screw it. We’re off to eBay. Look at that! Temperature controllers out the wazoo! Hmm. We are trying to do this cheaply, but with some assurances it’ll actually work. Let’s skip used equipment and focus on new parts. I can get a temperature controller with included thermocouple for $31.95 plus shipping. Hell, yeah.

Okay, here’s a snag. The max current we can pull with the relay circuit in the controller is 3A. As we know from doing our quick and dubious math, we need about 8 amps. So the included relay ain’t gonna work. Shit. However, our controller also has a DC trigger out. The controller puts out some voltage for the on state and drops to nothing for the off state. Let’s see, 8V out. That will drive a solid state relay quite nicely, won’t it? Well, hell, we’re already at eBay in another window. How much for an SSR? $8.99 for a 25A relay that triggers off 3-32 VDC. The price even includes shipping from some Chinese factory town. Sweet!

Now, we need a box to hold our controller, relay, and associated wiring in. I found yet another score on eBay: a PVC enclosure with aluminum back plate already installed for $25. Slightly used, but since it’s just a box to hold parts, I can live with that. Since we’ve got the back plate in the enclosure, we can dispense with a separate heat sink for the SSR.

So let’s see, we’ve got the heating elements, a way to control them, and a box to hold stuff. Are we missing anything? Oh, yeah, a pump. Well, I’ve got to go by the pet store and pick up some turtle supplies. I’ll see what’s available in the way of a pump and get back to you guys when the parts get here.

* Actually done in MathCad. If anybody wants to see the math, let me know.

**I did all the electrical calcs as straight DC, since an immersion heater is as close to a purely resistive load as uou're going to find in an AC circuit. Sloppy, yes, but I'm not an EE.

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