Do slow cookers require liquid to operate?

It's really no different from cooking something on a stove: if you have no liquid at all, things are probably going to start getting too hot, sticking on the bottom, and maybe burning. As long as there's some liquid, you'll be fine. The important thing is to add enough; if you keep having to open it to add more, it won't stay hot. This is basically the minimum level: enough that it won't boil dry in an hour or so. If you're cooking something really simple like a piece of meat, using too much water just provides more water for the flavor to get diluted into.

This is precisely what the first page you linked to says: if it's not full enough, it might all boil off. The writer is just way off about quantities. I suspect his cooker's lid doesn't fit well, or he's unnecessarily cooking on high. I don't think my slow cooker would boil dry from half full in 8 hours on low, and there's no reason to put it on high if I'm cooking that long.

The recipe at the second link isn't exactly empty - a can of cranberry sauce will provide a good amount of liquid. It also might not be a great recipe. On that note, I might suggest you look for more authoritative recipes. Perhaps check out a slow cooker cookbook from your library (these tend to be pretty common) to get an idea of the kinds of things you can do. (Of course, if all you want is a plain piece of meat, I guess you don't need much.)

America's Test Kitchen and other sources say the opposite of what's being offered as advice here. They recommend using very little liquid in the slow cooker, and that, paradoxically, more water actually dries out meats. Liquid in the slow cooker does not boil off. No moisture escapes. Yesterday I made the best corned beef I've had in a long time. Along with the 3.5 lb piece of meat, I added several potatoes, cabbage, carrots, celery, and onions, and only a half cup of liquid. Not only was the corned beef cooked perfectly after 8 hours on low,but the crockpot was nearly full of liquid.

My girlfriend found out by mistake that you don't need any liquid at all in a crockpot:

She turned it on and filled it with lamb, carrots, potatoes, and onions, but forgot to add water. After a few hours, we discovered that the juices from the meat and veggies were more than enough to not only cook everything to the point that it literally melted in your mouth but prevent anything from burning or sticking to the pot.

So no, you need not add any water when cooking with a crockpot, at least when you're cooking something with enough moisture of its own to cook in, which I suspect would be all meats and vegetables, as long as the lid works.

One of my favourite ways to cook a roast leg of lamb, if I am at work in the daytime, is to spray cooking oil on the leg, and place it with a whole garlic, straight into my porcelain crockpot(slow cooker). I never add liquid of any sort, as the meat will release its own steam. When I get home from work I put lamb leg into very hot oven, with a little oil and "roast" for 15 - 20 minutes. The meat falls off the bone, every time, and doesnt taste of anything other then roasted lamb.

I have told many people about this trick, and they all use it all the time now. At the moment today, I am doing the same method for a leg of Mutton, the house smells delicious !!

Also, you can also add potatoes or carrots etc, during the slow cooking time, and roast them with the lamb.

Easiest way I know for a weekday or anyday roast.

When there's less liquid, it's more of a braise. If you google 'braising in a slow cooker', you'll discover the people who use slow cookers with a lot less liquid.

I put halved onions on the bottom and add water.. the meat will sit on top of the onions and not submerged in the liquid.

I always brown my roasts under a broiler before adding to crockpot.....I never add any liquid and when it's done.....you'll always have a lot of liquid in your crockpot.

When I cook ribs or oxtails in the crock pot, I use a half cup of liquid (my secret: the broth from reconstituted shiitake mushrooms) and a little chopped onion and cook them FROM FROZEN on low overnight. (If the ribs are too big for the slow cooker, i put them in a covered roasting pan in the oven at 200°.) The flavor and color are delicious, as though they'd been browned first. Then, if it's ribs, I slather barbecue sauce on them and toss them in the oven on high for a few minutes. They're amazing. The oxtail, and any chuck I put in with it for family members who don't like the texture of oxtail, come out perfect; I cook the veggies on the stove in the rendered liquid from the meat and serve with barley, rice, or mashed potatoes. I cook frozen chicken parts without adding any liquid but only vegetables on the bottom and Cajun seasoning on top--5 or 6 hours on high, or low if the chicken isn't frozen. Easy and yummy and has plenty of its own liquid.

Because the temperature is low, there is not enough build up of really hot steam to escape the container to diminish the amount of liquid. By the time the small amount of moisture does rise to hit the lid, it forms droplets that fall back to start the process again. My problem is that the meat is really steamed, rather than broiled/roasted. When I add vegies the result is the same - steamed soft rather than roasted.

The hottest a slow cooker on low gets is about 160 degrees and on high about 180 degrees. This does not reach the boiling point. Use very small amount of liquid if you want to. When you BBQ a brisket you just put it on the pit at about 180 to 210 degrees or so with just dry heat and let it cook for 15 to 18 hours. I think brisket cooked this way is very tender and tasty. Enjoy.