There was lots of related information at the following sites:
"Camera Storage Tips", Robert Monaghan, <http://medfmt.8k.com/mf/storage.html>.
"Preventing Fungus and Mildew Attacks on Cameras and Lenses", Robert Monaghan,
<http://medfmt.8k.com/mf/fungus.html>.
Monaghan was not specific about numerical humidity ranges, so I did a little
poking around on Google using the search term "optics fungus relative
humidity". Above 70% RH (relative humidity), fungus grows like crazy,
especially if it's warm. Basically, fungus won't grow below 40% RH, but below
30% RH, static electricity becomes a big problem, so the target range is 30% to
40% RH.
How do we compute the effect of raising the temperature on RH? The precise
definition of relative humidity is the ratio of the actual partial pressure of
water to the saturated vapor pressure of water at the same temperature. The
presence of air is not required, although it is traditional where zuiks live.
<http://www.natmus.dk/cons/tp/coolfilm/rh_def.htm> These folk, art
conservators employed by museums, are sworn enemies of fungus. The critical
equation follows:
Saturation vapour pressure, ps, in Pascals: Ps = 610.78 *exp( t / ( t + 238.3
) *17.2694 )
where t is the temperature in degrees Celsius. A web calculator that solves
this equation is at <http://www.natmus.dk/cons/tp/atmcalc/atmoclc1.htm>. Just
enter the temperature and click the button as directed.
Let's solve this equation at a few common temperatures: 20 C -> 2326 Pa. 25C
-> 3148 Pa. 30 C -> 4212 Pa. 35 C -> 5577 Pa. 40 C -> 7309 Pa. 45 C -> 9489
Pa.
For the record, standard atmospheric pressure is defined as 101,300 Pa. Water
boils when the vapor pressure exceeds atmospheric pressure.
Let's assume we are living in a tropical clime, where the temperature is 30 C
(86 F) and the relative humidity is 90%. (I used to live in Baltimore, MD, and
this happened regularly in the dog days of Summer.) The saturation pressure of
water in the air at this temperature is 4212 pascals, but we are at only 900f
saturation, so the actual vapor pressure is (0.90)(4212)= 3790.8, or 3791
pascals.
If we take a cabinet full of this air and heat it by five degrees, to 35 C (95
F), the saturation pressure is 5577 pascals. Now, heating the air did not
change the amount of water in it, so the actual vapor pressure of the water is
still 3791 pascals, so the RH is now 3791/5577= 68%. This is still too high,
so let's raise the temperature another five degrees, to 40 C (104 F). The RH
is now 3791/7309= 52%. This is workable, albeit slightly RH higher than we
would like. Let's go to 45 C (113 F), where the RH drops to 3791/9489= 40%,
right on target for RH, but maybe a bit hot for storing cameras and film.
As for controlling the temperature in a cabinet, one can simply try various
sized incandescent lamps in the cabinet and measure the temperature rise, or
one can use an ordinary residental thermostat rated for 110 VAC or 220 VAC to
control a lamp. It may be necessary to use a relay to control the lamp
circuit, as the turn-on surge of a lamp is easily ten times the steady-state
current, and the surge may cause the thermostat contacts to weld together.
Electric supply companies sell self-powered solid-state "relays" intended to
allow a low-voltage (10 VAC) circuit to control a lamp of some hundreds of
watts, far more than is likely to be needed. (I used one of these relays to
make a closet light that comes on when the door is opened. Very convenient.
For safety reasons, the door switch must be low voltage.
The necessary wattage will depend on how well the cabinet is thermally
insulated. An old refrigerator would work well, as it's well sealed and well
insulated. It's also not the first place a burglar would think to look.
Anyway, the above will allow people to decide if a heated cabinet makes sense
in their climate.
Depending on where one lives, the tight cabinet with some cannisters of
dessicant may be easier. For film, it's probably the best way regardless.
Joe Gwinn
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