Zeers and refrigeration

Time for my annual blog post! Yeah, I know, I hardly ever post any more :P. Recently, I've been reading up on fridges and freezers -- I've been thinking of getting a deep freezer. While I was doing that, I read about a really simple invention, called a "zeer." In northern Africa, the weather is hot, and there isn't much refrigeration, so vegetables often go rotten within 2 or 3 days. A few years ago, a schoolteacher came up with the idea of taking a clay pot, and putting a smaller clay pot inside of it. You then fill the space between the two pots with sand, and then run water over the sand. Last of all, you put a wet piece of cloth over the top. Thanks to evaporative cooling, you get no-power-required refrigeration! It's very simple, but it makes vegetables last many times longer, and makes the markets both more hygienic and the produce nicer. Here's an article about it.

Deep freezers have a bunch of advantages, but the one that actually got me thinking about it was discovering that sushi-grade fish is (surprise!) always frozen before being thawed out for serving. In fact, it legally must be frozen to an extremely cold temperature, since such freezing destroys parasites in the fish. Now, shipping fresh fish is kind of impossible, but frozen fish? That can be done! But unless I use it within a few hours of delivery, I'd need to keep it frozen, and that means a heavy-duty, extremely cold freezer.

Unfortunately, I can't find any realistic specifications on chest freezers. It looks like some of them can go as low as -20 °F, which is cold enough, but I can never find a temperature range and a particular model listed in the same place. It looks like the dials on the sides of most deep freezers are labeled "1" through "6," rather than anything useful.

I've heard quite a few horror stories about the hazards of homemade sushi, mind you, so I'm trying to be Very Sure.