Re: gEDA-user: Conformal coatings, PCA water-proofing

[Bob Paddock <bob.paddock@xxxxxxxxx>]

> It is quite surprising that coating a PCA in silicone would not provide
> a water proof seal.  But based on what I have recently read, you are
> right that conformal coatings are permeable to water.

It is a mater of how long it takes.

> What about potting the circuit rather than applying a conformal coating?
> Could that provide a true water proof hermetic seal?  For instance,
> suppose I wanted to immerse a temperature measurement circuit in a
> container of water for extended periods.  Could it be potted in
> silicone or some other commonly available substance to protect it from
> water?

Nothing beats a welded box with gasketed connectors.
I've seen water wik up cables fifty feet in the air, got a shower from
one when I disconnected the connector.
I've seen potting wik as well.  Water always wins, it is just a
question of how long it takes.
The other thing to watch for is pressure differentials from inside to
outside the box.  Changes in pressure cause flexing that act like a
pump
to suck water in.  I've seen membrane switches turned into balloons,
when being transported from a mile below ground to a mile above
ground.
Gortex can help with that problem.  Like all things in this line of
work it is all trade offs.


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