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

GandalfG8 at aol.com GandalfG8 at aol.com
Mon May 14 23:01:55 UTC 2012


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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