A simple way to know your temperature is right on the mark.
I have read many posts in film photography subreddits about “what is wrong with my film”, the user getting unexpected results that look like underexposure or strange color shift. While most of these can be racked up to human error during the shooting of film, there is another unsuspecting culprit that could be to blame.
Most people may expect that thermometers are accurately calibrated right out of the box, but sadly this is not usually the case. I’ll keep my discussion here to dial thermometers that are ubiquitous in darkrooms as these are the most easy to calibrate for repeatable results.
When developing film, even a one or two degrees off can have significant impact on the process, both in B&W and color developing. A thermometer reading temperatures too low will result in thin negatives, or if calibrated too high resulting in high density and sometimes color shift.
How to get it just right? Easy as ice. Fortunately it is quite easy to make a water bath that will be exactly 32 degrees. A slurry mixture of ice (crushed is best) with just enough water to make the bath homogeneous is the trick. Submersing the thermometer in the center of that bath will render a reading of exactly 32F (0C). Or it SHOULD. Dial thermometers usually have a hexagonal fitting on the back that can be held with a wrench or screwdriver, allowing the thermometer housing with the temperature scale to be rotated. Simply “dial in” 32F (0C) with the thermometer in the ice bath, and voila!, A calibrated thermometer.
Be sure to not touch the thermometer to the bottom of the container or to its sides, keeping in the middle of the bath by either holding the housing with your hand, or resting the thermometer through a hole in a ruler or other instrument. Now you can be confident your processes are indeed at the right temperature.
- gregory gammon / http://www.vinylfye.com
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