Julian,
At 8:09 PM +0100 10/12/02, Julian Davies wrote:
>Not to worry. Calcium chloride is a solid, with properties similar to
>sodium chloride (table salt). These melt somewhere just short of a red
>heat.
>
>Joe
>
>
>Thanks Joe. It's some time since I used my physical chemistry studies, so
>they're a little oxidised.
>Does it make any difference that the drying properties of the substance
>must(?) result in a calcium chloride solution lurking in the micro - porous
>clay structure? I just had a nagging doubt / confusion about the
>independence of the components of a solution when it comes to vapour
>pressures and all that.
I doubt that the clay affects the drying effect, except perhaps to slow down
the maximum rate of moisture absorption.
The purpose of the clay is probably exactly to absorb the calcium chloride
solution, so it doesn't dribble out all over the place. So, the calcium
chloride *does* lurk in the micro-pores, and this is a Good Thing.
My recollection is that vapor pressures are totally independent, and the total
pressure is the sum of the individual vapor pressures. (I don't recall that
solutes in liquid solution work that way, but it would take some research to
sort out.)
But it really doesn't matter, as neither clay nor calcium chloride have
significant vapor pressures at temperatures that ordinary camera equipment can
survive, let alone the users of such cameras.
Remember that the dryout temperature is 400 degrees F. This will bake any
volatile component out of the granules, so when returned to ordinary
temperatures, nothing more will outgas.
The bottom line is that there will not be calcium chloride vapors (or anything
else) condensing on the camera gear, even in tropical climes, so it suffices to
contain the dust.
Joe
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