Chuck Norcutt wrote:
> But it it's hermetically sealed why does one ever have to change the
> dessicant?
Chuck:
As far as I can tell, it's a combination of things:
1. You do open the cases from time to time, and the dessicant's capacity
for humidity adsorption isn't unlimited;
2. There is some humidity clinging (for lack of a better term) to the
metal and glass (and interior non-sealed air spaces) of the lenses; and
3. Silica gel dessicant seems to "auto-reverse" its adsorption by
outgassing every so often.
There's also the issue of adsorption while they're cooling down after an
oven treatment (so they never go back into your cases "fully-charged,"
so to speak), and I have no idea of the efficacy or accuracy of the
indicator dots. I do notice that if I'm in and out of the cases
relatively frequently, I'll only get a month or so worth of protection
according to the indicator dot before I have to recharge. If I leave
'em alone, they can last many months. They never last forever, though.
I like 'em 'cause they don't require power on a regular basis -- if
that's never a concern, don't go with that solution.
And as for the quality of the seal on the boxes, as far as I can tell,
they're watertight. I've dunked 'em as tests, and I can't get 'em to
leak under a foot of water. I have no idea how well they resist
pressure differentials of atmosphere.
Garth
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