[TowerTalk] TowerTalk Digest, Vol 122, Issue 58

[Jim Lux]

On 2/26/13 2:33 PM, Patrick Greenlee wrote:
> Would someone please explain to me why you can seal a water pipe made of
> PVC and withstand over 100 PSI but not be able to seal a PVC pipe
> against 0.0 PSI because it has wires in it?

because you can't seal where the wires go in..
I used to build a lot of oil insulated HV gear, and my one take home 
lesson is: "oil always leaks".

The tiniest hole or porosity is enough to let air in and out, and if you 
think about it, a 100 foot run of 4" diameter conduit is a fair volume 
(about 9 cubic feet).  An air pressure change from 29 to 30 inches of 
mercury results in a change of 1/30th of that volume.
The daily temperature change, say from 50 to 78F is a change in absolute 
temp of 10K out of 300K, another 1/30 factor.

So a daily exchange of half a cubic foot or so isn't unreasonable to 
expect..


Sure some regions of the USA
> have high humidity so a little moisture will get into the pipe before it
> is sealed but not huge amounts.  If you have 100% relative humidity and
> want to have a dry interior of your pipe, drilling holes in it won't dry
> it out but will admit moisture.  Purge the pipe with CO2 from an
> extinguisher, or dehumidified shop air, or a bottle of dry nitrogen, a
> SCUBA tank,  or ... and then seal it.  Really, water doesn't penetrate
> PVC pipe very well if the joints have been properly assembled. This
> leaves just plugging the ends and sealing them with an appropriate
> adhesive sealant such as GE II silicone rubber or Excell (silicone
> alternative.)
>
> The above assumes you seal all terminations of all the wires so that
> there is NO path for water ingress. I assert that if the above is
> accomplished in a workman like manner there will not be water intruding
> into the conduit. Come on, this isn't rocket surgery..... or brain
> science.... ;) ;)

It is tough to get a truly airtight seal with cables.  There's flow 
through stranded wire, through the coax shield braid, etc.