Another from the Highlands!

Discussion in 'Say Hello & Introduce Yourself' started by Tim Parkin, Dec 22, 2017.

  1. Ian Grant

    Ian Grant Active Member Registered User

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    David, my thoughts are of heat exchange, a coiled copper tube passing tap water through a small header tank of tempered water controlled by aquarium heater(s), My current darkroom is down the end of my garden so has no hot water, I cope with a kettle.

    However I could do with a better controlled warmer water supply. I actually have 2 or 3 circuit boards with quite accurate temperature controllers from when I used to build gold refineries and these would work extremely well, if I can find the circuit diagrams and wiring notes, these would control any type of heater element.

    Keeping Tim's chemicals at a reasonable temperature might need a low fi approach like a seed tray warmer.

    Ian
     
  2. David M

    David M Active Member Registered User

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    I think the print washer industry has a lot to answer for, in making people believe that continuous flow is essential. It's intuitively attractive to imagine the flow scrubbing the surface of the film or paper, but that's not what happens after the first rinse.
    The chemicals diffuse out and form a very dilute solution. If this is removed entirely, then the next round of diffusion in fresh water produces even lower saturation.
    Imagine, for simplicity, that the film is entirely made of fixer solution. Let's guess it's 0.2mm thick. A volume of water 5"x4"x 10mm (apologies for the mixed units) will come to equilibrium with the concentration reduced to one fiftieth of the original fixer. A further fresh bath would reduce this to one two-thousand-and-five hundredth of the original strength of the fixer.
    This is oversimplified nonsense of course. The emulsion layer might be a tenth of this and will only be holding the fixer that has soaked into the gelatin. Shall we guess ten percent of that? We would not wait for perfect equilibrium in practice, so my figure are over-optimistic.
    Even if my mental arithmetic is out by orders of magnitude, we can still see how the concentration drops rapidly with successive fresh new baths.
    I don't know if you've ever tested the effectiveness of continuous flow. You need a transparent tank of some sort and some ink. I was amazed at the length of time it took to entirely clear all traces of ink as there always seem to be pockets that resist the flow. I did this with my print washer (when I used one) so this may not be strictly applicable to film.
     

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