[TowerTalk] Ew: PL-259s Going Open?

[Kim Elmore]

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.