c41 film processing unit

Hi There,

I'm a games programmer with 20 years experience in java & C. I'm also a very keen photographer and got high end digital cameras but I miss film development and is looking into the feasibility of developing a low cost c41 / black & white development unit.

the unit would perform

  • automate film processing 3-5 stages depending on film development type
  • agitation
  • heat and maintain film development chemicals into precise temperatures (developer, bleach, fixer, stabiliser,water)
  • dynamically change processing times for each chemical component / process (under, over expose)

Most of the parts of the unit are fairly mechanical and I feel confident I can manage to build that, however when it comes to
water heating and maintaining temperatures I'm not clear where I should start.

so my question is : what would the best route be to control / maintain the water temperature in a 1 - 2 litre plastic component (rectangular, 20cm in height)

  1. thermo electric units ?
  2. water pumps using hot and cold water to regulate temperature

is there any alternatives. I have not managed to find something on the web, though I have found a lot of water measuring tools / controllers.

Kind Regards,
Lennart

What's your temperature range and tolerance?

Hi Chagrin thanks for your reply,

I need a pretty acurate (calibrated) thermometer. The standard deviation for C41 development is 37,8 degrees C +/- 0,5. Some kits have slightly more tolerance. The K54 (Amaloco - C41) has + 0,5 -1 degrees C tolerance and for less contrast you can go to - 1,5 degrees C lower temperature.

So temperature range : 20 - 40 degrees C
tolerance : +/- 0,5

Kind Regards,
Lennart

Have done some research tonight and think this should do the trick

1x water block (WBA-162-055)
2x 44x44mm TEC (cooling on one side and heating on the other)
1x water pump connected to water block and main liquid container

of course I need the thermostat etc, but as far as the heating solutions goes I think this is the way I'm going at the moment.

/Len

A DS18B20 is spec'd at less than .5C at your temperature range. A small motor driver will allow you to reverse and control the current through the peltier; ideally the motor controller should be capable of the full amperage requirement of the peltier but in your case, at almost room temperatures, you more than likely won't need to be driving it with that much power. You'd need to calculate how quickly the film bath cools off to understand how much power you'll need.

The PID library will help you with controlling the peltier based on readings from your temperature probe. The code and examples are a little hard to grasp (it seems to operate almost magically) but I'll guarantee it will be the simplest and best way to do it in the long run.

A DC pump can be pretty easily controlled with a mosfet; an AC pump should use a relay.

Thanks Chagrin,

Yes sounds really good. Thanks for that. Stayed up well to late last night searching water blocks, peltier coolers, plastic welding, water pumps...dreaming about plastic welding...how much fun isn't all this !

Thanks again for your reply.

Kind Regards,
Lennart