Cooking: immersible thermocouple?  (sourcing q)

I've wanted for years to make my own oven that heats well and keeps a stable temperature. But that project can wait until after I move (in September) and have a garage again.

In the mean time, I've wanted to get started with a PID controller project, so I figured I'd try a PID controller to turn a hot pot on and off for 'sous vide'-style cooking. But I've come up against my usual block when starting these projects: figuring out what parts to buy. Most pages I've found assume you have a stronger basis in this sort of thing ("you just need a thermocouple and a driver, and a solid state relay!"--great, but when digikey has 5,000 of each I get confused fast).

So, I found this nice Instructable and figured for a first time out I could go with the AD595CQ (probably AQ as the CQ seems to be out everywhere). But I'm trying to take the temperature of a water bath, and most type K thermocouples I can find are air-type. The nearest thing I could find is this, but not only is it mildly pricey, it's out of stock until mid-May.

Any suggestions? I'd prefer something "food-safe" but it's probably not necessary for the sous vide project as the food is bagged. Can you use "non-immersible" thermocouples and just put them in a bag too?

For a PID project, you SHOULD use RTD sensor platinum100, is good (the golden standard) For an example: (just a random page for info,don't buy there) http://www.picotech.com/applications/pt100.html

These sensors are most defenetly FOOD SAFE and availible in a convenient probe, (mostly).

then find a cheap instrument amplifier IC to interface with it

David [edit]Omega tends to have good products ranging from cheap to pricy http://www.omega.com/ppt/pptsc.asp?ref=PR-10

RTD elements can also be purchased with resistances of 200, 500, 1000, and 2000 [ch937] at 0°C. because of their higher resistances at 0°C, they provide more resistance change per degree, allowing for greater resolution.[/edit]

Personally, for this kind of project where you don't need sub-degree accuracy (you're cooking, not setting up a science experiment), I would hack a inexpensive food-grade digital thermometer for the thermister; what you could do is parallel the output of the themister with the Arduino, then start a bath of water from frozen (freeze the probe in the water; get one that can clip to the side of a pot), and bring it up to boiling. Set up the Arduino to read the thermister, and note the reading and the temperature on the probe. You will develop a "curve" of readings; it may be flat, but I doubt it - I think most thermisters are logarithmic, not linear. You could also avoid this if you can find out the thermister's spec sheet, but this might be hard to do from a purchased digital thermometer. Your other alternative is to find a thermister with the right temperature window for food cooking (probably 0C to 250C would be large enough) - so you can get the spec sheet.

You don't need a thermocouple - thermocouples are typically used where you need high accuracy, plus high-temperature measurements (think measuring the temperature of molten glass or such).

Lastly - I have to question the need for such a thing period; most professional chefs don't worry about the need for such an oven - why do you think this is?

:)

Lastly - I have to question the need for such a thing period; most professional chefs don't worry about the need for such an oven - why do you think this is?

I didn't think there was a real need for a serial-CAD-device-to-3d-HID-gaming-controller interface, but I did it anyway... the primary result is to learn something, and having something semi-practical to come out of it is a nice hook.

Actually, for sous vide, professional chefs do worry about the need for a temperature-controlled circulating bath with a few degrees of accuracy, and such a device is not inexpensively available; one for home cooks costs a ridiculous $450. I don't actually have that much interest in sous vide cooking beyond the curiosity (although it would have advantages for entertaining and some things), but it makes a good candidate project for learning about PID control and thermosensing.

The oven project is not truly necessary at all, but the last two ovens I have had in my houses have been not only wretchedly inaccurate (off by tens of degrees) but had very sharp temperature gradients. The electronics in most ovens are really minimal, from the ones I've taken apart--basically just big insulated boxes with heating elements. I'm curious to see if I can take a cheap/discarded oven, hack an arduino and some sensors to it, and make something that performs better than a top-of-the-line model, with really silly extras (IP-addressable web server with real-time temperature graph? Why not?)

I would hack a inexpensive food-grade digital thermometer for the thermister; what you could do is parallel the output of the themister with the Arduino,

Hmm, there's a good thought, esp since I have one of those handy with plug-in thermistor probe. I'm guessing I'd have to make a voltage divider circuit a la here. May play with that and see what happens.

I still sorta like the thermocouple idea, just for the fun of overengineering something, but this would be fun too.