What is the best way to regulate the temperature of developer and fixer whilst tray developing prints?
Black & White materials are usually processed in fluids that center on 20°C (68°F). This was considered “typical” room temperature in Europe. Most color films and papers are designed to operate at a much higher temperature. The best method to adjust and hold the temperature of the fluids of the process is the water bath”. This can be an inexpensive plastic tray. The bottled fluids of the process are placed in this tray, half submerged. The temperature of the surrounding water bath is adjusted as needed to bring the bottled fluids to the correct temperature. Once the correct temperature is reached, we monitor it with an accurate thermometer.
On the web, you can find and purchase laboratory equipment designed to set and keep the water bath at the desired temperature. However, such devices are generally not needed. All that is necessity is to pre-prepare all the fluids that will be used and pour then into trays and then process the films or papers soon after.
If laboratory accuracy is required, you can invest in a circulating tempering system. These are expensive devices that both heat and cool controlled by a thermostat.