I dropped my ancient trusty Kodak mercury thermometer a few months back and finally replaced it with one from Patterson. I then used it to check my dial thermometers to see if they had stayed “calibrated” (I had reason to suspect they had not). I found that one was two degrees higher than the mercury reading, and the other was 3 degrees higher. This means my last few negatives have been under developed by using a cooler temperature than the 70 degrees I was shooting for. However since I use a minimal agitation technique the lower temperature was not obvious. I also made  a trip to a kitchen and bath store and found a digital thermometer on sale. It ranges from -40 to +450 degrees in 1/10th increments. So I brought it home and tried it against the mercury. It gave me a precise readout of 70.4 degrees for my kitchen cold tap water. I compared this to the mercury and could see the mercury was between 70 and 72 but not very precise and subject to +or- a degree depending on angle viewed.  I then adjusted my dial thermometers by turning the nut on the back so that they read 70.5 degrees and were calibrated to the digital.

I have long heard it doesn’t matter which thermometer you use as long as it is consistent. I have now seen first hand that the dial thermometers may expand or contract in the heat and cold of the storage area. While digital precision is not needed in the darkroom for paper printing, it adds that extra bit of repeat ability for film developing. I would imagine for color and slide printing it is more important to maintain the correct higher temperatures for proper color of the images. Just food for thought if you are struggling to get consistent results.