[time-nuts] wwvb weak on east coast especially when the pre-amps under wa...

[K3WRY at aol.com]

If you can keep the boards in a vertical mount position, and they have been 
 sprayed with a conformal coating, the heat from the components and the 
coating  will keep any moisture from forming on the boards in a vertical 
position.   We do this in several products we supply to the military.
 
Dr Joe Palsa k3wry
 
 
In a message dated 5/14/2012 9:03:13 P.M. Eastern Daylight Time,  
GandalfG8 at aol.com writes:

The  other option of course is to pressurise the box with dry air to ensure 
  
a positive pressure differential, such that the net flow is always  
outwards 
at  all points, but it's probably easier just to provide a  drain hole:-)

However, whilst a drain hole will prevent the build up of  a lake inside 
the 
enclosure it still doesn't prevent condensation forming  on circuit boards, 
and  powered circuit boards and condensation don't  really go well  
together.

As per earlier comments, it's quite  difficult to keep any externally  
mounted enclosure totally moisture  free, so it's much easier to accept the 
 
inevitable and allow for  it.

In a past life I designed quite a few circuit boards that   were required 
to 
be fitted in externally mounted vented enclosures, so  not  a great deal of 
pressurisation there then:-), and I usually  specified that  both sides 
should be sprayed with a plastic coating  following final test.

I can't remember now exactly what this stuff was  called, but it  was 
readily available in the UK from both RS and  Farnell as an aerosol plastic 
 spray 
that provided a good barrier but  was a bit more flexible than the  usual 
MOD 
spec conformal  coatings.
It melted easily under a soldering iron, albeit with a foul   pong:-), so 
reworking was no problem, and resisted moisture  remarkably  well.....

problem  solved:-)

Nigel
GM8PZR





In a message dated  14/05/2012 23:10:30 GMT Daylight Time,  
arnold.tibus at gmx.de  writes:

The only  solutions I think:
Apply air pressure tight  boxes having a breathing hole  an the bottom,
mount the
box that no  rain and water can penetrate from  the top or sides. If the
hole is  big enough,
eg. 2mm, no pressure  difference is possible and no  pumping effect will
occur.
(If the hole is  too wide, small animals  may penetrate).
Or,
when using a pressure tight  box, it must be  stiff and sealed to
withstand under all
temperature  conditions  more then 1 bar/ 100 kPa. Do not forget that  all
feed
throughs  must be of real hermetic type, normal coaxial  connectors are
not  tight!
Don't route cables directly in, because no  cable braid or mesh  is   vapor
tight.
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