[filmscanners] Re: What can you advise

["Bob Frost" <bob@frost.name>]

Henning,

Having just read my reply to you, I think I can simplify the matter further
still for any photographers who are still with us.

You said:

> Actually, Bob, 100% relative humidity at 5° C is equal to about 29%
> relative humidity at 25° C.

Fine; it is in the sense that each probably holds equal WEIGHTS of water (I
don't have the precise figures to hand).

But from a mold's point of view, 100% RH at 5 degrees is perhaps a trifle
cold, but the air is saturated, condensation will occur, and the mold will
grow all over your film, given time.

However, at 25 degrees and 29% humidity, despite the equal amount of water
in the air, the air can hold many times that amount of water before it
becomes saturated, and no mold will grow on your film because any water will
evaporate and the mold spores will not germinate or will become dessicated
if they land on your film.

You might think, that is clear from the RH values - 100% and 29% - it is
obvious that 29% has less water than 100%. But my other example shows this
is not always true - 80% RH at 25 degrees is equal to 30% at 5 degrees FROM
THE MOLDS POINT OF VIEW. The only way of directly comparing the dryness of
the air from a mold's point of view is to measure the amount that it is
below the maximum water-holding capacity of the air. While Relative Humidity
allows comparisons to be made at any one temperature, you must measure
Vapour Pressure Deficit to compare dryness between one temperature and
another.

I hope that is clearer.

Bob Frost.


----- Original Message -----
From: "Henning Wulff" <henningw@archiphoto.com>
> >
> >The problem is that last word - temperature. As an example, 80% relative
> >humidity at 25 degrees C is equal to 30% at 5 degrees 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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