I'd never heard of sous vide before coming to this site, but all the posts about it here have made me want to try it. I've seen the beer cooler method, which I also want to try, but, for a smaller scale test, is it possible to do it in a crock pot? What would the differences in cooking times and techniques be?
Yes you can. Some of the best sous-vide I've had has been in a crock-pot.
You'll need a crock-pot with a manual (analogue) dial. What you do is set the dial to maximum and use a PID controller (found ~$20 on ebay).
Take a look at this article about hacking your slow cooker:
Sous-vide by definition requires control of the temperature (Beer Cooler does not control, but holds well). However, you can still do this without extra logic. All you need is a good thermometer and time:
Fill your crock-pot to 70% with room temperature water, set your Crock-Pot to the lowest setting, and leave it for 2-3 hours. Thermo-dynamics laws dictate that you will reach an equilibrium with time. If you're bored you can keep a log of the temperature change over time and stop when you notice it doesn't change much any more (it'll be helpful for next cooking session).
Measure the water temperature and check against where you need to be (125, 140, etc). Adjust the dial and throw more time at it until the crock-pot reaches equilibrium again. Repeat until you have reached and sustained your desired temperature. Make sure your crock-pot is not near an open window with draft to mess with the crock-pot (i.e. keep the room temperature steady).
Weigh your food and put it in the rock-pot. Take out the same amount of water from the existing water in the crock-pot so that the total weight of the crock-pot is the same.
This isn't hard if you use the metric system. Each Millilitre of water weighs 1 gram. So for 1lb, take out 454ml or essentially two cups.
Now you can use the same timing and temperature rules as everyone else.
PS: For the 'ballistic' method, I.E. the beer cooler, you can measure the temperature loss over time of water in the cooler to land you perfectly where you want to be. Chef Kenji says a 'couple of degrees higher'. But you can predict the drop over your cooking time as well as the initial temperature. And roughly speaking: Mass Of Food x Food Temperature + Mass of Water * Water Temperature = Total Mass * Final Water bath temperature.
Short answer: no, you cannot do sous-vide in a slow cooker without extra equipment or modifications.
Sous vide cooking tends to be at moderately low temperatures, well controlled, and for longer periods of time.
Crock pots or slow cookers do not precisely control the temperature of the contents of the pot. In fact, many of them do not even have a thermometer to measure the temperature in the pot--they rely on the idea that they put out enough energy to keep the full pot up to a safe (but not precisely regulated) temperature all of the time. Even their low cooking temperature may be too high for sous vide applications.
There are some DYI hacker projects that take a crock pot and add a thermometer probe and a logic board to a crock pot to create a low cost simple sous-vide setup. However, even in these cases, there will be no agitator or fan to create convection in the liquid within the crock pot, although they may be suitable for some cookery projects. You might google "crock pot sous vide hack" to see many instructional guides such as this one from PopSci.
Another low end option (as you mention) to get started for very simple sous-vide cookery which will not be for many, many hours is to use a beer cooler (see Kenji Alt's Serious Eats article), and simply fill it with water (slightly above your desired temperature), and put the food in that.
For maximum safety and accuracy, however, eventually you are going to want a set up which provides:
For cooking in the home kitchen, I find the 7 quart oval CrockPot to be an excellent vessel for sous vide when attached to an external temperature controller. You must use the "simple" knob-controlled type (mine has low-high-warm settings available. The models with digital controls and temperature probes are not yet programmed for sous vide cooking, although I have hopes that some day they will be.
I have used both a thermistor-based thermostat (Johnson Controls A419) and a platinum (PT-100) temperature sensor and PID controller (JLD 612 with solid-state relay) successfully. Most do-it-yourself sous-vide home cooks like myself would assume the PID controller to be the preferred equipment, but I have found I prefer the thermostat. The water temperature oscillates up and down about two degrees using the thermostat, and many would fear that loss of control, but my results trump those concerns. The core of the meat remains at a stable temperature that is the average water temperature, and even very near the surface the temperature does not track the oscillations. I use an aquarium air pump and air stones to provide circulation.
I do take control of the temperature measurement accuracy by calibrating against accurate and trusted digital thermometers. The digital fever thermometers sold in pharmacies are accurate, and allow calibration of the thermistor or PT-100 sensor at a temperature range of about 100 F.
The CrockPot holds larger roasts quite comfortably, and with heated walls requires no separation of the meat from the walls by racks as is practiced with commercial cookers with cold walls. I insist on using an aquarium airstone and pump to cause water to circulate, and believe this is why my results are good.
The advantages of the CrockPot as I use it over, for example, the Sous Vide Supreme, are:
1) Easier cleaning -- hard water spots dissolve away with a few drops of lemon juice, and the glass pottery can also be removed and washed in a dishwasher. This means a lot when, as I did, the cook actually practices sous vide daily for home meals!
2) Comfortable size -- no wasted corners and cold walls means more space for larger roasts but the cooker can still be lifted without strain while filled with water.
3) Dual utility -- I can use it as a CrockPot instead of a sous vide cooker when I want to.
4) Probably the biggest advantage that the CrockPot with A419 thermostat has over PID-controller based cookers is that the thermostat recovers unattended from short interruptions in the electrical power. The JLD612 as with most PID controllers, would allow meats cooking while I am at work to rot on the pot. I suspect that a power outage of perhaps three seconds may result in the same problem with many commercial cookers.
1) This is an "off-label" use of the CrockPot. I expect fairly that my CrockPot will lose a heating element to all the temperature cycling some day, though after about 500 meals it is still going strong. Rival cannot be expected to take responsibility for such failures when their products are subjected to the unanticipated abuse. I humbly submit that the low cost of replacement of the CrockPot outweighs this risk. I often cook roasts that cost more than the CrockPot, and don't object to regarding it as an expendable like the ZipLock freezer bags I use with it, and the aquarium tubing that I replace every few months.
2) Low prestige -- I can't show this cooker to those who've spent $450-$3000 for higher-end products and fairly expect them to understand that I'd truly prefer this to their set-up at any price. I can't explain to the owner of a silent Sous Vide Supreme that the sound of bubbling in the CrockPot bothers no one, but means my 7-qt cooker can cook more than their 12-qt cooker, with my 85 watts bringing big roasts up to temperature faster than their 400 Watts. I can't explain to them that the 2-degree temperature oscillations from the A419 are no factor in flavor or safety, that they are at greater risk from the cold walls of their cooker than I. I have to let them feel smug.
My final analysis: the ability to pre-cook the meat in advance of crunch time allows me, or any home cook that uses sous vide, to put dinner on the table with much less effort after I get home from work. If I were forced to buy a high end sous vide cooker to get this benefit, it would be a small price to pay. The tenderizing and the consistency of preparing sous vide meats has made mealtimes a great pleasure for me. But my choice of equipment has numerous advantages that have made cooking with my setup a true pleasure.
Most of my mealtimes involve fairly familiar comfort foods that cook all day while I am at work. 24 hour tri-tip roasts, 12 hour chicken breasts -- boneless and skinless -- that I'd prefer cooking for four hours, and 24 hour pork loin come to mind. This results in the kind of mealtime convenience required for me to be able to even attempt cooking myself. If I did not have reliable regulated temperature my sous vide cooking would be limited to weekends, when I'd prefer to be eating out. I estimate I've cooked nearly 550 sous vide entrees now. I could no longer be satisfied cooking without the technique. But the beer cooler method requires constant attention that limits it's use to weekends and short cooking times. I recommend it for 35 minute fish filets, 3-4 hour chicken, or 3-4 hour beef tenderloin. Any longer cooking times will discourage its use. Now, a beer cooler with a PID controller and an electric water heater element could be good method...
Sous vide is a technique created by professional chefs that had the idea to re-purpose an immersion circulator traditionally used in laboratories where incredibly precise temperature are required. The same temperature requirement of precision is needed in the kitchen when cooking sous vide.
Using a cooler would work, but only as an insulated vessel to hold the water. However, the water bath or vessel, is not the issue - you could use a plastic bin (as many chefs do). The problem is reaching and maintaining a very precise temperature throughout the water bath, for the total duration of cooking time. Hence, making the crock pot idea not really viable for cooking sous vide.
This is not to mention, of course, the whole "sous vide" (under vacuum) thing...it is also very important to create a full vacuum in a perfectly sealed vacuum bag. If you are interested, the best value professional equipment I know is at: www.vacuum-packer.com