[TowerTalk] more pulling cables

[Grant Saviers]

Ground temps are usually lower than where the cables enter the shack and 
the ground temp swings are larger if there is a real winter at the QTH.  
So, temp cycling pulls wet air into the conduit and condensation 
results.  Then there is the likely but small ambient pressure 
differences from wind at the two ends.  Once the condensation is there, 
there is little opportunity for it to get out.  Not a "major problem" if 
the cables are rated for direct burial, as are all buried conduit 
electrical cables.

An analagous problem exists for air dielectric coax and waveguides, so 
they are pressurized above ambient with dry nitrogen or very dry air.  
An alternative for conduits of getting a dry enough air supply if the 
buried conduit is below freezing is a proposition requiring a 
refrigerated dryer (gets to 35 deg F or so dew point) and ping pong 
recycling dessicant dryers (can get to -60 deg F).  So we live with some 
water in the conduit.  Maybe a low point drain to a small pump can 
reduce the amount of water,  but I think it will still be "wet" inside.

If water in the conduit is a real problem or for academic interest or 
for the lunatic fringe ham (hi hi) with "concerns", Tessco sells 
packaged dryers for compressed air for about $2 to $3K and the conduit 
needs to be sealed up at the ends as best as possible. 
http://www.tessco.com/products/displayProducts.do?groupId=415&subgroupId=10

Grant KZ1W


On 2/26/2013 8:57 AM, Al Kozakiewicz wrote:
> Two things:
>
> 1. I find it difficult to believe that condensation is a major problem.  The volume of water in the water vapor contained in the air trapped in a conduit is pretty small even if the air was warm and saturated when sealed in.  There shouldn't be an enormous amount of air "turnover" to replenish the water vapor - it's not like there are large volumes of air circulating in a closed (though not airtight) space.  There are literally hundreds of thousands of miles of buried electrical conduit in the world with no special care taken to prevent condensation that one would have to expect a high failure rate if it was a major problem.
>
> 2.  Maybe in the desert it makes sense to drill holes in perfectly watertight conduit.  But anyplace that has a normal rainfall and soil that retains any moisture, all that would accomplish is to make sure the conduit is always filled with water.
>
> IMO. YMMV.  LSMFT.
>
> Al
> AB2ZY
>
> ________________________________________
>
snip