One of the mistakes folks make it to attempt to make the box completely
sealed. You really can't seal a box effectively without going to extreme
measures. It gets expensive to do this. Because of changes in
temperature and atmospheric pressure, air will find some way to come it
and out, dragging humidity and perhaps water with it. It is, as you say,
best to accept the fact that a box will breathe and to deal effectively
with that breathing.
It is best to place the vent in a place that will naturally be sheltered
from drips or will not draw drips into the enclosure. Your tube that ran
up the back of the enclosure is a good example. Next, it is important to
make the "exhale" of the box expel any water somehow has that found its
way into the box. Your vents in the bottom of the enclosure are a good
example of that, (but they would also tend draw in drips from the
corners outside.)
A good design might be to put a small rail around the bottom edge of the
box, and put the vents on up on the inside of that rail. Adding a
desiccant in-line with the vent can really keep the inside dry, but you
have to service/change the desiccant regularly, which most users won't
do. What does really help is to keep the interior dead volume to a
minimum so it will breathe as little as possible. If the power is
available, a small heater makes a huge difference by keeping the
interior above the dew point, stopping condensation. But that can be an
unacceptable loss of energy.
Potting the components to completely eliminate the dead volume around
the parts is a very effective approach. It makes the device
unserviceable, however, and it sometimes makes the heat build up
unmanageable. Conformal coating is probably the most cost effective method.
Bill D.
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