Re: Hunidity - RH

[Katharine Thayer <kthayer@pacifier.com>]

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,
Katharine