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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