Jim, I don't disagree with your math but I do disagree with your statement
that you can't seal the conduit. Silicone rubber around snug fitting holes
in a PVC pipe cap are sufficient but if you want to get super serious you
can use compression type bulkhead feed through fittings, the kind with
rubber corks with holes for the wire work very well. There are others made
with O rings that seal good enough to be used on oceanographic instruments
intended to stay water tight at great depths. My 3 inch buried conduit is
dry (I have a pull string and can swab the bore but have never found water.)
The PVC pipe caps can be installed with an external bead of silicone caulk
or Excel rather than PVC cement. This gives a good seal but can be easily
cut with a utility knife to gain access and reassembled with more caulk.
Patrick AF5ck
-----Original Message-----
From: Jim Lux
Sent: Tuesday, February 26, 2013 10:50 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] TowerTalk Digest, Vol 122, Issue 58
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.
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