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

[Joe Gwinn]

At 10:49 AM +0000 5/15/12, time-nuts-request at febo.com wrote:
>Date: Tue, 15 May 2012 08:59:04 +0200
>From: Attila Kinali <attila at kinali.ch>
>To: Discussion of precise time and frequency measurement
>	<time-nuts at febo.com>
>Subject: Re: [time-nuts] wwvb weak on east coast especially when the
>	pre-amps under water.
>Message-ID: <20120515085904.19669d7edd8b95454f176036 at kinali.ch>
>Content-Type: text/plain; charset=US-ASCII
>
>On Mon, 14 May 2012 18:01:01 -0400
>Joseph M Gwinn <gwinn at raytheon.com> wrote:
>
>>  Modern outdoor enclosures use a filter of some kind, but the underlying
>>  principle is the same.
>
>I don't know what other types are around, but we use vents with
>a gore-tex foil over them. Keeps water out but lets the case breath.
>This prevents any pressure build up, which would then start to suck
>water in from the seal.

Gore-tex blocks liquid water (and dust), but allows water vapor to 
pass, so one can still get pumping and the accumulation of condensed 
water.  Where are your enclosures used, and how exposed are they?


As for the other methods, so far the following have been discussed:

Box with long tube, where the tube volume exceeds the tidal volume of 
the enclosure, so outside air never manages to get to the enclosure. 
Typically, the tube is plugged with some cotton wool, to keep insects 
out.

Box with short tube and filter/dessicant.  The Gore-tex film is a 
filter, but not a dessicant.  A box with medium tube and filter plus 
dessicant at the box end can be very effective, the medium tube 
reducing the rate at which the dessicant is exhausted.

Box with short tube that opens on conditioned space.

Totally hermetic box.  Very effective, but very hard to do in 
practice, unless the box is small and strong.

Box where positive pressure is maintained using dry air, so all leaks 
are outward and the dew point is never reached inside the box.

Joe Gwinn