One reason, perhaps the reason, my home colour developing never really was spot on may well have been lack of temperature control.
I have tried using a bath of tempered water for all the chemicals of the Tetenal C-41 kit but it is not that easy really. How long should the PET bottles with the chemicals be in water to get to the right temperature inside? Five minutes? Fifteen? Fifty? Either way it is not easy to keep a constant temperature of the water for prolonged time.
I usually start with 45C water, drop the bottles into that and let them be there until temp has fallen to 38 or 39C. That is when I start develop.
Recently I set upon making a more proper temp controller. From Conrad.de was bought a temp controller with relay output and a solid state relay capable of switching no less that 40A of 240VAC! A small plastic housing was made and the parts installed. Next came the hunt for a heating element of some 400 – 600 watts. And that is what I have yet to find. Until then I got a 200W aquarium heater. It actually had a temp control built-in so I had a go with that just for fun.
Note: A power of 200W is way too little to be really useful. It takes ages to heat the water in the first place. Though it does keep an already tempered water at the suggested temperature all right.
The quick setup in the sink consists of the heater at the far end, the pump at the bottom and a precise thermometer for process control floating on a bit of foam.
Apparently heating aquariums is a pretty sloppy business as fish are too stupid to really care, while developing colour is a different business where exact control is said to be needed: The hysteresis of the built-in regulator was something like +/- 2C. And the scale only went up to 32 C so some slight massacring of the scalewas necessary to be able to increase its range. It seems to work even afterwards:)
A trial run was made with the aquarium heater and a 220V pump intended for making water splash around in ponds. The water was wildly circulated and everything was guaranteed to have the same temperature after a few hours. The actual temperature was supervised with a Taylor 9842 digital thermometer calibrated at 0C. It took several hours of heating before entering a steady state of 38-something C.
The procedure was a usual, 3m45s dev (due to overused developer), and 10m blix. As the blix runs to exhaustion I am no longer picky with the suggested 4 minute period. I tried to keep the Paterson tank submerged in the 38C water at all times possible. It all went very well and the colour casts I usually get were gone! I am onto something here!
The end result is that the setup works but I look forward to a more precise heating and control apparatus in the future. It would also be pleasing with the heater, pump and controller as a single unit instead of the present cable mess in the kitchen!