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

Alan Melia alan.melia at btinternet.com
Mon May 14 23:32:11 UTC 2012


Well almost Nigel, if you look at molecule mean velocities they are always
able to diffuse down a concentration gradient (i.e from wet into dry)
despite a small reverse presure gradient. It just takes longer under those
conditions :-))....which is often "enough"

Alan G3NYK
----- Original Message ----- 
From: <GandalfG8 at aol.com>
To: <time-nuts at febo.com>
Sent: Tuesday, May 15, 2012 12:01 AM
Subject: Re: [time-nuts] wwvb weak on east coast especially when thepre-amps
under wa...


> 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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