Peter-
I have no dispute with your knowledge of weather, but you have not included
all the phenomena; radiation heating (from the sun) is a significant
contributor to the pumping action, and diurnal temperature changed cause
condensation as well.
There is a phenomenon called "Permeability pumping" that contributes to the
problem. It happens to chambers that are closed, but have porous closures
through which vapor will move but water will not. The cycling of pressure
and temperature, in phase, around the dew point causes water to accumulate
in the chamber. This can be a real problem with repeater antennae enclosed
in Fiberglass tubes. This is where I first saw it. However, it can happen
with 9913 where the porous access is the braid at the connecter.
Bill-W4BSG
----- Original Message -----
From: "Peter Voelpel" <df3kv@t-online.de>
To: <towertalk@contesting.com>
Sent: Sunday, December 21, 2008 5:01 PM
Subject: Re: [TowerTalk] Ew: PL-259s Going Open?
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Kim Elmore
> Sent: Sonntag, 21. Dezember 2008 23:31
> To: towertalk@contesting.com
> Subject: [TowerTalk] Ew: PL-259s Going Open?
>
> As a bit of additional info, I haven't direct-buried any 9913: i's all in
> conduit. That said, there's no guarantee that the conduit has remained
> perfectly dry. That said, I have seen no evidence whatsoever that the coax
> itself has suffered any water ingress, and I've even attempted to draw
> water
> through the coax like a straw (to quote Yukon
> Cornelius: "Nothin'").
>
> Now, a comment: I've heard stories of changes in barometric pressure
> and/or
> temperature causing air-dielectric cable to "breathe." I've found no
> evidence of that in may case, and I've looked pretty carefully. As a
> meteorologist, I can say that atmospheric pressure changes by only about
> 3.5% from highest to lowest pressure and under isothermal conditions, this
> will cause a volume change of 3.5%. As for ingress via water vapor, there
> is
> actually very little water in the air. Under saturated conditions (100%
> RH)
> at 20 C, the mixing ratio is 15 g H2O per kg of dry air (about 15 cc). At
> an
> RH of about 50%, there's about 8 g H2O per kg dry air (about 8 cc). A kg
> of
> dry air at STP has a volume of about 1 cubic meter. I don't know what the
> contained volume of 9913 is, but I'll bet it takes a very long length to
> contain 1 cu m. To actually condense out all the water would require
> cooling
> the air to a very cold temperature (about 0 F would do most of the job).
>
> Given that the induced volume change is very small, water ingress due to
> vapor drawn in by pressure changes (or temperature changes, for that
> matter)
> is probably negligible. Now, I'm obviously neglecting any hygroscopic
> concerns. But, if an open end of the coax effectively sits in water and
> the
> other end is well sealed, it could act as a straw and draw water into the
> open end. However, there's a limit to how much water could be drawn in, so
> not even this effect could fill a tube with water. In fact, if you drew a
> vacuum on the open end, you could support a water column of only about 34
> feet. So, to get much water into the coax, there must be a couple of leaks
> someplace, with a way for air to escape and be displaced by water. Under
> that circumstance, you could completely fill the coax with water.
>
> _______________________________________________
>
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>
> My experience with air dielectric cable is different.
> When not run vertically condensed water will collect and short the cable
> at
> the lowest point of the horizontal run if not pressurized.
> It took 5 years to happen on a 150m run of 7/8" cable at DF0CG/DR1A, of
> course some hours before the start of a contest.
>
> 73
> Peter
>
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