How do I cook a 65-degree egg in my own house?

August 24, 2009 8:35 PM

sous-vide-Filter: I have a pot of water and I can buy an immersion heater or aquarium heater, but how do I control it to maintain an arbitrary temperature 40-100C within 1 degree, for cheap? External themostats for homes and microbreweries and aquaria seems to top out at 32C, and cost upwards of 50$. Scientific water baths and ovens cost $800+. And my oven is not accurate enough.

The immediate purpose of this setup will be to cook an egg at 65C, but after that I may branch out to meat. For this reason, a big vat of boiling methanol is not appropriate, even though it would maintain 65C quite nicely.

The ideal solution will cost less than $50-$100 and involve lots of equipment I already own or which will be useful to have around afterwards.

posted by d. z. wang to technology (13 comments total)

Probe thermometer?

Doesnt help you control the temp, but reduces bogus oven readings and the chaos introduced by opening the oven a bunch.
posted by shownomercy at 8:39 PM on August 24

Thermostats for espresso machines usually fall within the 90-160C range, most commonly 100C and 145C. They also seldom fail, so an otherwise broken espresso machine should have 2-3 functioning thermostats in it (on the boiler). New, they're $3-$10 depending on the manufacturer and temperature setting; my Ascaso catalog shows ~20 between 85-180C and also a 0-90C variable version with a long probe and dial adjuster.
posted by bizwank at 8:50 PM on August 24

$800!!!

Cheapy controllers here.

Thermocouples here.

Just hook it all up with whatever immersion heater (or whatever) you want.
posted by Confess, Fletch at 8:54 PM on August 24

Here you go: $50 digital temperature controller with thermocouple. This should be able to control just about any heating element, e.g. a hotplate from a thrift store.
posted by Rhomboid at 9:49 PM on August 24

Step by step instructions, ca. $150. Maybe you can economize!
posted by kenko at 10:44 PM on August 24

@bizwank: That 0-90C variable version sounds like exactly what I'm looking for! I tried the acaso website but it seems like their catalog is only accessible to vendors. I didn't have the login credentials. Googling for retailers didn't find anything either. Do you know of any place to buy just one, online? Also, would I just wire this in series with the heating element?

If I can't find this espresso thermostat, Rhomboid's is looking like the best solution for its price. I don't need a full process controller (and wouldn't have the electronics knowledge to do anything with one either).

Or, for another approach, I do have a computer lying around, and I'm much more comfortable in software than hardware. Would it be possible to hook a thermometer into a computer, and then have the computer control a heating element? A quick Google did not turn up any USB thermometers cheaper than $39 for the Go!Temp.
posted by d. z. wang at 10:57 PM on August 24

You'd need some kind of relay control circuit if you wanted to use a computer, because heating elements use tons of current. A quick search turns up various kits but they generally are probably undersized -- those kits are generally targeted for people that want to turn on and off their christmas lights or whatever. That one is rated at only 10A at 120V which might be too small depending on the type of heating element that you choose. And besides, you'd need some electronics background to wire it up. (The $50 unit I linked above is going to require a slight bit of DIY as well; most likely it's got standard SPDT relay terminals which just means you put the NO and C in series with the element.)
posted by Rhomboid at 11:44 PM on August 24

eBay has them by the score for $50 or less. Many have integral relays, and if you can find one with solid state relay outputs, you'll likely get sufficient power rating to run a heater. It's not a huge amount of power.

Several considerations: The amount of water you are trying to maintain is a factor, since more water will heat slower with a given thermal input. Unless you have a PID controller, you'll get overshoot and undershoot around your setpoint, and even then, you'll get some. Still water will have hot and cold spots in it, and to make it a thermally homgenous medium, you should agitate it. Otherwise you will control the temp at the point where the measurement is made, which again, depending on how you probe things, may be a legitimate average, but for large quantity of water, there will be variations, particularly as you approach the heater.

IN short, get a controller from ebay, agitate the water, use a small amount of water, and size your heater to the themal load presented.

If you want stability, aim for a duty cycle of 50% or so. If you want response time, aim for a duty cycle of 20%. If you want precision, aim for a duty cycle of 80% or so. Thermal control is fun, and like any other automation, it involves a lot of factors to consider to get exactly what you want.

The ideal controller would be one that was continuous and not bang-bang (also called deadband controller.) This would use DC or phase controlled AC for modulating the heater input. Most don't. THey use 100% on/off (hence the bang-bang moniker) control. I don't recall seeing one of these in my travels, but it would be easy to build (for me!) if this is a complex process you are trying to control.
posted by FauxScot at 1:15 AM on August 25 [1 favorite]

I didn't know if the methanol part was a joke or not, but if you can boil it at 65C, why not make a double boiler with methanol in the bottom part and water in the top? You might lose a couple of degrees between the bottom and top boiler but it'd be pretty close, right? I could be totally wrong on this, but it was the first thing that popped into my head.
posted by bengarland at 6:28 AM on August 25

FauxScot is right that you'll need an agitator.

In my experience with vats of hot water, you won't really need any sort of complex control system to get it to the right temperature and keep it there, if you're willing to put a couple minutes into setting it manually. The basic technique is to crank up the heating element and watch the thermocouple readout until you hit a little below your target temperature, then back it down slowly until the heat loss to the atmosphere = the heat gain from your heating element. Monitor and adjust as needed.

Relevant experience: I once designed a portable device to circulate hot water at a specific temperature through some copper tubes.
posted by muddgirl at 7:11 AM on August 25

Forgot to add that you want to locate the thermocouple as far from the heating element as possible.
posted by muddgirl at 7:14 AM on August 25

I don't know if you are interested in building some control electronics, but the guys over at SparkFun built a toaster oven that they can control to 2+- degrees. Maybe you can adapt the electronics they designed.
posted by gregr at 8:45 AM on August 25 [1 favorite]

I would skip the fancy temperature controls. The cheap and easy way to solve this problem is with thermal mass. Specifically, I would suggest a double boiler made from the biggest pot you have available - like a saucepot inside a turkey fryer. It'll take a while to get up to the desired temperature, but once it's there it will be easy to keep it there. The more water you use, the more stable the temperature will be.
posted by Anoplura at 11:49 AM on August 25