That wasn't Peter, it was me. Peter is innocent!
Yes, radiational heating was ignored in my analysis and that can heat
the coax to very high temperatures without some sort of ventilation.
I'd guess the stuff could hit 160 deg F or more on a hot Summer day,
now that I think about it.
Permeability pumping is certainly real (Gore-Tex (tm) uses it). But
I'm not convinced that this is always the case with 9913 and water.
As I'm led to understand it, liquid water can certainly enter through
the connectors, and I'm certain that permeability pumping doesn't
explain water running out of coax into a shack. Repeater antennas are
subject to severe conditions: extended periods in cloud, where the
air is at least saturated and sometimes supersaturated for hours on
end (something we do not see near the surface), long periods of
direct sun exposure, and high winds that will generate large
dynamically induced, rapidly changing pressure gradients about any
object. Not to mention the vibrational problems. Exposed antenna
sites are extremely demanding on anything and I'm amazed at how well
the things we put up there do.
But, regardless, that's not the problem I face here and now. I still
have the mysterious, spontaneous failure of a PL-259 that was outside
to wrestle with.
Kim Elmore N5OP
06:20 PM 12/21/2008, Bill Aycock wrote:
>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.
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
> > 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
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
>
>
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