RJS has made an important observation. Years ago when boats floated away more than a few of my dollars, I learned that it was not necessary to buy marine grade parts at a boat store, paying a premium. There are quite a few darkroom items that fit this bill also.

I have a conventional dial Beseler thermometer, and it is used inline to check water line temperature. For solutions, etcetera, I use a Polder digital oven thermometer. Reads in tenths, has an alarm to warn when a certain temperature has been reached, and a repeating timer as well. Its calibration was tested against a Beckman Ph/Temp rig and was within slightly one-quarter of a degree of dead on all along the range. Price: under $30 at places like bed, bath, and beyond.

For quicky measurements, one of the little digital cooking thermometers works well, and are usually within one-half a degree accuracy. Price: under $10 at most grocery stores and big box retailers.