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  1. #21

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    Quote Originally Posted by Loris Medici View Post
    Actually, I think absolute water content of air (and paper) is more important:
    Water content of air at 20C (68F) / 40% RH = ~15g (which is saturation humidity per kg of air at 20C) x 40 / 100 = 6g (per kg of air)
    Water content of air at 30C (86F) / 75% RH = ~28g (same as above, but for 30C) x 75 / 100 = 21g (per kg of air)

    As you can see, the water content of air changed more than 3x while the RH change was less than 2x! (1.875x, to be exact...)

    Another example:
    Water content of air at 20C (68F) / 50% RH = ~15g (which is saturation humidity per kg of air at 20C) x 50 / 100 = 7.5g (per kg of air)
    Water content of air at 25C (77F) / 40% RH = ~24g (same as above, but for 25C) x 40 / 100 = 9.6g (per kg of air)

    Now, things get interesting here; even if 40% RH (at 25C) is numerically smaller than 50% RH (at 20C), the absolute moisture content of air is more in that case, therefore your emulsion will be slightly faster!!! (Even if the RH figure was lower...)

    Moral of the story: Don't be fooled by just taking RH into account; you have to pair that with temperature, in order to be able to make a good educated guess...

    Regards,
    Loris.
    Fair enough, but the temperature is inside and doesn't vary - always at 72% within a degree or 2 and the RH is also measured indoors.

  2. #22
    Ian Leake's Avatar
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    Quote Originally Posted by timgray@rogers.com View Post
    Fair enough, but the temperature is inside and doesn't vary - always at 72% within a degree or 2 and the RH is also measured indoors.
    What's important is the moisture content of the coated paper when it is being exposed to UV. Room temperature and humidity only effect the rate at which the coated paper dries in still air. They are therefore almost irrelevant to the look of your print if you force dry the paper (e.g. with a hair dryer or fan), control paper humidity with a humidity chamber, or have very long exposures (which cause the paper to dry out under the heat of the lights/sun). If they are fairly consistent then you can ignore them.


    Afterthought:

    If you air dry your paper in still air then room temperature/humidity will effect your drying time so being aware of them is a good thing. Likewise if you move your darkroom to a new location your timings may need to change due to the new ambient conditions. And extremes of temperature and humidity are unpleasant to work in. But it's important to understand that your objective needs to be to control the moisture content of your coated paper NOT to control room humidity.
    Last edited by Ian Leake; 09-11-2010 at 10:07 AM. Click to view previous post history. Reason: Afterthought...

  3. #23

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    Ian, indeed what's important is the moisture content of the paper. And as stated in the afterthought, if you dry your paper in still air, the environmental conditions affect the vapor pressure of water, therefore the remaining moisture levels in paper, at time of the exposure. I don't use a hairdryer - except for cyanotype, where I leave the sensitizer soak in for at least 10 minutes and then force dry... - with any of the processes I do, because I found I get better prints (= better dmax / punch and hue) and more consistent results when I do otherwise. I also protect the paper by placing 2 mil thick impermeable sheets of mylar below and above it, therefore I don't loose moisture during exposure. (If you loose moisture during exposure, with pop pd, you'll get warm corners / edges vs. cooler center - which is awful 99% of the cases...)

    Regards,
    Loris.

  4. #24

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    Quote Originally Posted by timgray@rogers.com View Post
    Fair enough, but the temperature is inside and doesn't vary - always at 72% within a degree or 2 and the RH is also measured indoors.
    Fair enough, if your temperature is always within +- 2F of 72F, then only taking into account the RH is perfectly OK. (As you have already experienced...) In my case, the indoors temperature of my studio will vary between 18-31C (64-89F) depending on the season, and the RH will vary between 30%-75% - independent of the season. Therefore, I have to be a little more fastidious...

    Regards,
    Loris.

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