For chemical process, Fahrenheit scale is needed. For film and paper processing, the normal temperature for B/W film processing is 68F (20C), for color film 100F (37.8C) and color paper 92F (33.3C). Centigrade can work well for B/W but not color because it is difficult to read for decimals. Of course, we can round them up to 38 instead of 37.8 etc. if our thermometer is for Centigrade scale only or without one tenth scale. Nowadays, colored alcohol can react as fast as mercury and is easier to read.

Another choice is the digital thermometer. The display is good but the price may be higher. Some of this design may not be water-proof.

For personal darkroom, we do not need a calibrated thermometer. Consistency is the most important for this kind of use. It is better to stick to one with proven result.

THERMOMETER