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
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