U of S | Mailing List Archive | alt-photo-process-l | Re: Hunidity - RH

Re: Hunidity - RH

On Sep 29, 2006, at 6:36 AM, Jeffrey D. Mathias wrote:

Keep in mind also that RH is just as much a function of pressure as temperature. That said, RH is what one wants to watch (as well as temperature and pressure.) This is because RH indicates the partial pressure of the water vapor which is what will determine if water is going into or out of the paper from or to the air.

Note too that the water vapor can move from or to the paper at a pretty good speed (a few minutes). I have placed a paper onto a scale and watched the weight change (heavier or lighter) as the water vapor came to equilibrium.
Ah, just what we need, another variable. :--) But your point is well taken.

I also agree with your note that it doesn't take long for the coating (here of course I'm talking about gum) to absorb or give up moisture. I wouldn't expect to see a lot of difference in my exposures, since I dry my gum coating with a hair dryer and print immediately when it's dry; for these little test strips it's probably not much more than a minute from the first brushstroke to exposure. So I would expect the moisture content of my coating to stay fairly constant regardless of the RH. But still, I consistently see the differences that I've reported in these discussions. If I air-dried my paper, I would expect to see much greater differences in exposure adjustments consequent to varying moisture content.

Thanks for the information,

  • References:
    • Hunidity - RH
      • From: "Jeffrey D. Mathias" <jeffrey.d.mathias@att.net>