Re: [Leica] Off topic: Vaults and Humidity?

[Malcolm McCullough <blayne@mbox2.singnet.com.sg>]

>How do you keep the humidity low inside a closed vault or safe?  The space
>inside is usually tiny.....a dehumidifier is not practical.....
>How can we vent the moisture away?
>
>I am currently using a big canister of silica gel.....
>
>
>Frank
> 

The models I have work in the following way:
A tiny solid-state heat pump creates a small cold spot onto which moisture
condenses. The condensate drains out through a small tube and evaporates in
the reject heat outside the cabinet. The rate of water
production/evaporation is very low, as is the cooling power of the heat
pump (c 7 watts). The cooling power must be low, otherwise the temperature
in the cabinet would fall and all your gear would get a layer of
condensation each time you opened the door. The RH is maintained at about
50%. The rate of water removal is low - an empty cabinet takes about two or
three days to come to 50% from 90%. I keep a number of absorbent items
(leather cases, cardboard filter wallets etc.) in each compartment to act
as buffers for moisture.

And yes - they are enormously expensive when compared to domestic fridges,
but not too bad when compared to the similar solid state cooling units sold
for yachts, caravans and portable coolboxes etc.

They are very different from room (refridgeration/reheat) dehumidifiers in
terms of the amount of water they produce and the way in which it is taken
out of the system, but otherwise they work in a similar way, scaled down
very much. Back in the UK I have my darkroom in the cellar. The
dehumidifier (refridgeration/reheat type) collects about two litres (half a
gallon or thereabouts) of water per day.

I think that the problem with silica gel drying is that the RH is not
controllable. Some materials, film emulsion included, become stiff and
brittle at low humidities - and silica gel is capable of creating very dry
atmospheres.

Regards,
Malcolm
Singapore