I notice on the
Fotoimpex page it says:
"Precision: 1% (equals +/- 0,2° at 38°)
when immersed at least 15 cm deep in liquid"
Most of the 500 mL containers which I use to develop film are less than 15 cm deep. Likewise, it might be difficult to get 15 cm of immersion in a darkroom tray for paper development unless you drop the thermometer, dial and all, into the tray?
It looks like a fine thermometer, and I'd like to have one. But users should be aware of the fine print when using any thermometer.
As for calibration, a local university darkroom has about a dozen dial-type thermometers. I put them all in a liter of 20*C water to see how they compared. As I recall there was a several degree difference between the highest and the lowest. Keep in mind these had been used by students for some time, so possibly not handled with the greatest of care.
I bought my own dial type thermometer (a Samigon), which I calibrated against a digital thermometer. It is easy to change the calibration of the dial-type by holding a nut with a wrench and rotating the dial. My digital thermometer -- a ThermoWorks ThermoPop2 -- is specified to be accurate ±1.0°F (±0.5°C) from 14 to 208°F (–10 to 98°C), and is NIST-traceable. According to ThermoWorks, I can get an accurate reading with only about 0.25-inch (6.4 mm) immersed in a solution.
The worst things about the digital are: it keeps turning itself off to save the battery, and it is not that quick or easy to switch the readout between Centigrade to Fahrenheit. The dial thermometer is always on, and I can read either C or F at a glance.