The floaty ones are calibrated extremely poorly. But they have the advantage of being really cheap and stocked in quantity at the LFS, so you can check them all out and pick one with a "consensus" reading. But who knows, maybe one of the outliers is the one that's right. I'll never get another one of those though after breaking two, one in the tank. Fortunately the little balls in mine were ferrous, so I was able to harvest them with a cleaning magnet. I think I got most of them out, but what a pain.
The digital ones aren't necessarily calibrated any better, so again, I guess it would be best to put a bunch together in the same water and find the consensus. But the big advantage with the digital ones are the features. Most of them have IN and OUT (though I've never found them labelled with fish tank semantics, so IN means in the room and OUT means at the probe, whereas it would seem more intuitive to have IN mean the probe in the tank and OUT be the room). I have one with alarms like Nate, though mine are settable, not preset. But I also have one with MAX and MIN temps, both for IN and OUT. This is even more useful in my opinion than an alarm feature. If I'm home, I already know whether there's a temperature issue in the apartment, because I'm uncomfortable. So the alarm really only tells me about heater failure I guess. But it's great to come home and be able to check whether there were any temperature issues in my absence. The best part of it is that this little gem was something like $5 at Circuit City in the junky gadgets section.
However, the original question was about accuracy and precision. What I would love to find is a digital thermometer which has some sort of calibration mode. What exactly I'd calibrate it with I don't know, but even calibrating it to some consensus reading would be worthwhile.
--cn
