[volt-nuts] keeping water out of enclosures (was: Low-cost voltage reference questions)

Attila Kinali attila at kinali.ch
Fri Jan 22 16:10:53 EST 2016

On Sun, 29 Nov 2015 11:49:14 +0000
"Dr. David Kirkby (Kirkby Microwave Ltd)" <drkirkby at kirkbymicrowave.co.uk> wrote:

> A friend and I were thinking of developing antennas, which would be used
> outside, but enclosed in a fibreglass or similar tube.  I suggested
> flushing the antenna with nitrogen, then putting silica gel inside.

I know I am a bit late, but I'd like to comment anyways.

At my former job, we have built devices that have been installed
in high alpine environments and left there unattended for years.
The "technique" we used was standard aluminium casings with rubber seals
(similar to Hammond 1550W series), with a gore-tex breather hole and
filling all unsued space with silica gel.

First thing you need to know is, that it is impossible to keep an
housing completely airtight or rather watertight, as long as you
cannot control ambient temperature and pressure. The air pressure inside
will change and there will be a very slight breathing. If you don't give
it a safe way to breath, it will breath through the rubber seal. But this
breathing will be nasty: Breathing out the air from inside and sucking
in the (condensated) water droplets from the outside. Ie it will slowly
accumulate water. That's why we had do add a breather hole with a gore-tex
seal. You can change this to a long and narrow hole pointing downwards,
that should be equally good. If you make the hole volume large enough to
be larger than the worst case breathing volume, then you will never suck
in outside air. Depending on the environmental conditions you might still
need silca gel to keep any water vapor that diffused through the hole 
from condensating at unsuspecting electronics (and I assume that you
will need a lot of silca gel in the UK weather).

I am pretty sure, that the idea with the overpressure nitrogen would work,
but not with a fiberglas tube. These are not pressure tight enough. They
will leak out all overpressure within a couple of months (if not weeks).
For this to work, you'd at least need aluminium or stainless steel and
build the whole thing in the way how high vacuum chambers are built.
Nothing for the faint of heart.

If you reuse silica gel from bought electronics, you should bake it out
for a couple of hours, somewhere between 100°C and 150°C. (electrical
oven, not gas oven)

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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