[TowerTalk] Water in conduit

[Roger]

Please remove Digest from subject!

On 2/27/2013 5:28 PM, Patrick Greenlee wrote:
> 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

I said it before and I'll say it again:  Silicone rubber as in the RTVs 
is not moisture proog.  Water proog as in liquid? yes, but not for the 
vapor.

I say this as I worked for one of the major manufactures for many years.

  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 at 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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