Developer Temperature Test, Unheated Tank
C41 requires keeping the developer as close to the 100ºF/38ºC temperature as possible during the temperature-critical developing step. The remaining steps should be at the same temperature but are far less temperature critical. The same is true for processing E6 films.
Here is a simple experiment you should perform to get a good idea how this works. Use your developing tank with empty reels inside. Load the reels into the tank and fill with water at about 105ºF/40.6ºC or somewhat higher, cap the tank and allow it to sit for 2 minutes so that the tank and reels absorb heat from the water and come to a stable temperature.
In the meanwhile, prepare a liter dose of water at 100ºF/38ºC to simulate developer. Pour out the tempering water in the tank at the end of the warming period and add the 100ºF/38ºC water to the tank, install the lid and start a timer set to 3:15. Agitate occasionally to keep the temperature uniform to simulate developing.
At the end of the period, remove the lid from the tank and measure the temperature of the water. For a liquid-in-tube thermometer, start with the thermometer prewarmed to the approximate target temperature for faster registration. Depending on the room temperature, you might find that in the short time it has been in the preheated tank, that it has retained most of its original temperature.
If the temperature drop is quite small, then no compensation is required. But suppose that it drops from 100ºF to 98ºF. You could start with 101ºF that drops to 99ºF over the developing time. The average temperature over the developing period is 100ºF. The result should be approximately the same as if you had a constant 100ºF temperature for the entire 3:15 developing time.
If the room in which you do your developing is unusually cold, you might have to start at a higher temperature to compensate. By doing this experiment BEFORE you process your film at the room temperature you’re likely to use, you can get good results by preheating the tank loaded with exposed film in this manner and with minimal equipment.
By keeping notes of the experiments at various ambient temperatures, you can figure out the best starting temperature.
C41 Capacity
Tetenal and other makers used to include a temperature/time nomograph in the data sheets packed with each C41 kit (developer, blix, stabilizer). This was to compensate for probable temperature drop at various ambient temperatures. The older 1-liter Tetenal kits I used specified a capacity of 16 rolls per liter using the following developing step schedule at 100ºF/38ºC:
Rolls 1 – 4 at 3:15
Rolls 5 – 8 at 3:30
Rolls 9 – 12 at 3:45
Rolls 13 – 16 at 4:00
Then discard the exhausted chemicals.
Between processes, the developer and other chemicals are poured from the tank back into their respective storage bottles to form a uniform mixture with the remainder of the chemicals prior to processing the next batch of films.
This assumes waiting until you have 16 rolls of exposed film and will consume the entire capacity within a few days. C41 developer will begin to deteriorate once mixed. This proceeds slowly at first and accelerates with usage. If you don’t develop that much film, then you should scrap it much sooner, say, after processing 8 rolls.