Keep in mind that some of the bi-metal dial thermometers have an adjustment nut on the backside of the dial casing. These can be very handy if you are standardized on a specific reference thermometer in your work. Or as you can imagine, they can also be a royal pain in the butt if they unknowingly fall out of calibration while being banged around.
The large dial Weston thermometers have these adjustment nuts. I found that mine would need to be checked and adjusted frequently enough that I just made it a regular part of my development routine. Eventually I got tired of that extra step, so I calibrated my inexpensive glass Kodak Darkroom Thermometer to 68F/20C* against my reference and now use that. It never goes out of calibration. But I do need to be more careful.
* 68F/20C on my reference thermometer registers as 69.6F/20.9C on my Kodak instrument.