On Mon, Jul 03, 2006 at 09:16:51AM +0300, Vasile Surducan wrote:
> > > Even the boxes are hermetically sealed, what make you think will not be
> > > moisture inside ?  Huge temperature variations will create sweat
> > > inside a hermetical sealed box as well in an opened one.
> > > Critical temperature is 7-8C when the temperature is increasing from 0C.
> >
> > Not that I've ever done this sort of work but what's wrong with throwing
> > a couple bags of dessicant in the boxes? It's aparently good enough to
> > keep surface mount chips from going pop.
> 
> Sure, but as I said, the moisture is still there, now in the silica-gel pack.
> I've just want to point to a problem which exist, no matter what
> you're doing, except if some radical solutions are used, which are far
> away too difficult and too expensive.

True, and come to think of it, I wonder what it takes to get the
moisture *out* of a silica-gel pack... Could really screw up there.

> A hermetically sealed box with a PCB inside, running one day in open
> sun could have even water inside in the next morning.
> 
> One example (nothing in common with the topic).
> I've used a 250W 220V bulb as a flooding device for opening and closing a valve
> (something more or less identical with the classical closet reservoir).

Flooding device? You mean, you used it for it's buoyancy, not for any
electrical properties?

> The water temperature insidethe tank  was between 10C and 90C. The
> bulb works in this way one year. After one year inside the bulb I
> found water, filling about 10% from the bulb volume. The bulb WORKS
> after that when I supply it (with the bulb kept in a position where
> water does not touch the filament. Conclusion: the porosity of the
> bulb's glass allows bidirectional gas/fluid transfer. More or less on
> this principle, the glass PH-meter
> is built using a very porousive glass.

What can I say, WOW! Actually I've been doing a fair bit of research
into making hermetically sealed containers as I'd like to eventually
make some vacuum tubes for fun. They always say glass (and some metals)
are the gold standard in lack of permiability, but even they let helium
though. They also didn't say exactly what types of glass to use...

I'd be interested to try that experiment again.

> Why a hermetically sealed plastic box can't have the same behaviour?
> :)

Well above mentioned research said over and over again that plastics are
generally useless if you want a good hermetic seal.

-- 
pete@petertodd.ca http://www.petertodd.ca
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