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[filmscanners] Re: What can you advise?


We're both right I think. I believe you are saying that if you heat air at 5
degrees C and 30% RH up to 25 degrees, the RH will drop to 9%. What I was
trying to say, not totally successfully obviously, was that you cannot use
relative humidity (RH) to compare the DRYING EFFECT of air at different
temperatures. You have to use Vapour Pressure Deficit (Diffusion Pressure
Deficit or Saturation deficiency) measured in kPa, mBar, or mm Hg.

At the two temperatures I chose as examples (5 and 25 C) there will be a
maximum amount of water that can be held by the air, and at those max levels
the Relative Humidities will both be 100% and the VPD's will both be 0. If
the water content of the air is reduced at both temps to give VPD in each
case of 5 mm Hg (giving the same drying effect on anything containing
water), then the RH's will now be about 30% at 5 degrees and 80% at 25
degrees. These are the values I said were "equal" - equal in terms of drying
effect or VPD (but not in terms of total amount of water held, which is what
you were saying I think).

The VPD is the critical factor, not RH directly, from the point of plant,
animal, or fungal growth on films (getting back on-topic), as it is the
measure of the drying effect of the air. There is a useful ref on this at

It's a long, long time since I had to think about this!!

Do you agree now that we're both right?

Bob Frost.

----- Original Message -----
From: "Henning Wulff" <henningw@archiphoto.com>
> Actually, Bob, 100% relative humidity at 5 C is equal to about 29%
> relative humidity at 25 C.
> 30% relative humidity at 5 C is about 9% relative humidity at 25 C.
> I'm an architect (and architectural photographer) with special
> qualifications relating to moisture control in buildings, and the
> various aspects of moisture as applicable to construction and
> building science - no specialized knowledge of mycology, other than
> how it relates to buildings :-).

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