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: <[email protected]> To: <[email protected]> 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, > [email protected] 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. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
