[time-nuts] wwvb weak on east coast especially when thepre-amps under wa...
jfor at quikus.com
Mon May 14 23:39:59 UTC 2012
The military deals with this all the time.
Some equipment is housed in near hermetic, gasketed, metal enclosures with
bags of drying agent, like silica gel, or purpose built, screw in
dessicators, often with a humidity indicator. The HP 117A clock has one of
them on the front panel.
For larger volumes, like the "football" loop antennas used on WW II
aircraft ADFs, the units were sealed with gaskets and had a 1/4" or 5/16"
robber tubing connected to a cartridge dessicator filled with silica gel
that was pink when damp, blue when dry. These antennas had a tidal volume
of a gallon or two and breathed a lot in going from sea level to 25,000 -
You can easily mahe a sealed enclosure from a cast aluminum box, sealed
> 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
> under wa...
>> The other option of course is to pressurise the box with dry air to
>> a positive pressure differential, such that the net flow is always
>> 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
>> enclosure it still doesn't prevent condensation forming on circuit
>> and powered circuit boards and condensation don't really go well
>> As per earlier comments, it's quite difficult to keep any externally
>> mounted enclosure totally moisture free, so it's much easier to accept
>> inevitable and allow for it.
>> In a past life I designed quite a few circuit boards that were required
>> be fitted in externally mounted vented enclosures, so not a great deal
>> 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
>> that provided a good barrier but was a bit more flexible than the usual
>> 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:-)
>> 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
>> (If the hole is too wide, small animals may penetrate).
>> 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
>> 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
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