On 7/3/06, Peter Todd <pete@petertodd.ca> wrote:
> 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.

I've tried it two consecutive years on the same reservoir but I never
reproduce it again ! It was a defective bulb for sure.

Vasile
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