Here is a great post about slow-cookers. I use them all the time when we are busy (which is almost always). I love the information he gives here. He also has a great blog called How To Cook Like Your Grandmother. Check it out, it's full of great recipes and tips!
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I’m a fan of slow cookers, but lots of people have trouble with them. There’s a good reason, which most people don’t know: Slow cookers today aren’t nearly as slow as they used to be. Rather than risk lawsuits from people undercooking their chicken, the manufacturers have been raising the temperatures of their cookers for years.

The Low setting on many new cookers will cook faster than the High setting on some older ones. So if you’ve got a recipe your grandmother wrote based on her slow cooker, you’re going to overcook whatever you put in yours.

That’s not the only thing you need to understand to adjust recipes. Manufacturers don’t publish the details of how hot the “Lo” and “Hi” settings are. (They also don’t know how to spell “Low” and “High”, but that’s a different issue.) Actually, they can’t publish how hot those settings are, because they’re not controlled by a thermostat.

Designed for time, not temperature
Here’s the deal. Some slow cookers have a thermostat dial on them, like an electric skillet. These can be set to a temperature, and that’s how hot they’ll get. But most of them — and if yours just has High and Low, or maybe Hi-Med-Lo, yours is one of them — are based on how much power is sent to the heating coil. They are designed based on how long it takes to raise a certain amount of food to a certain temperature.

So for instance, the High setting on a 5-quart pot might be designed to bring 3-quarts of liquid from 60° to 185° in 2 hours — all temperatures Fahrenheit — while the Low setting will do it in 4 hours. (All these numbers are just for example. Your pot is sure to be different.) What that means is if you were to put 2 quarts of liquid in the pot, and it’s already at 85°, you’re going to overshoot “done” in about an hour. On the other hand if you load it to the brim with leftover soup right out of the fridge, it won’t be ready to serve for at least 4 hours.

In any case, Both High and Low stabilize at the same temperature, it is just a matter of how long it takes to reach the simmer point.

Get to know your slow cooker
The point of all this is that you have to adjust all your recipes to your slow cooker. No one else, unless they have the exact same model, can tell you how it’s going to work.

And even then it depends on how much you fill it up, and how cold or warm the ingredients are when you add them. None of this is really hard, but you have to pay attention, not follow a recipe exactly and expect it to come out right the first time.

But are they safe?
Mention slow cookers and someone is going to jump in with the figures on bacteria growth: Bacteria can grow if food spends 2 hours between 40° and 140°. True. But there are a few things you can do to make it safe.

First is to realize that any contamination on whole cuts of beef, like a roast, is likely to be on the surface. Bacteria generally do not penetrate into the meat. So brown the meat before putting it in the slow cooker. You should be doing this anyway for the best flavor. Browning will destroy any bacteria on the surface, as well as raising the starting temperature so that it doesn’t spend as long in the danger zone.

This doesn’t work for whole chickens or turkey pieces. According to Cooks Illustrated:

Bacteria can live inside a chicken, not just on the skin. Chicken parts can be browned to boost their temperature quickly, but this is not really an option for whole birds.

So for poultry you want to get it through the danger zone as quickly as possible. Start on High until the cooking liquid is over 140° before turning it down to Low.

For me, that defeats half the point of slow cookers. I like to set it and walk away. If I have to babysit the pot and check the temperature, I’d just as soon roast the chicken in the oven with a meat thermometer in it to tell me when it’s done.

Perfect use for them
For a really great recipe to start getting used to it, you could try slow cooker BBQ ribs. Because ribs are tough, it takes a long time to break down the connective tissue. It’s almost impossible to overdo them in a slow cooker. I know that’s like giving someone driving directions and saying, “You can’t miss it.” Someone will find a way.

But slow cookers are forgiving, and so are ribs. Give it a try and tell me how they came out.