Gary,
1. Keep on postin' you have a significant interested audience and the
others have delete buttons/keys or can program their email client to
send your stuff directly to the trash.
2. I get the same concern expressed by otherwise normal folk. They
tell me you will always get condensation in conduit. I have plenty of
4 inch PVC conduits and they have no water problem except one that was
broken due to improper backfill. I sleeved cables inside flexible
irrigation tubing before pulling in that conduit.
As the environment in which your conduit is located experiences heating
and cooling as well as changes in barometric pressure air is alternately
drawn into the unsealed ends and expelled. Moisture in that air may
condense inside the conduit and build to a level that would cause
problems. The really detailed worriers are concerned that even if the
conduits are sealed when a vacuum is created in the conduit air might
flow in the braid of coax or whatever and get into the conduit somehow
(or by other mysterious means.) I've heard a lot of low probability
mechanisms brought up as potential means of failure by folks often
recommending holes in the conduit to drain the water underground. To me
that is almost like leaving a hatch open in a submarine to drain out the
inevitable condensation. Later in this post I will detail an incident
involving a conduit with subterranean "drain holes."
With no holes in the underground portion of the conduit the rate of
condensation buildup would not be substantial in most cases. There are
various trade names for desiccants, Dryzit I believe is one I have
used. The little paper packages of silica gel that comes in so many
items can be saved, recharged in the oven and reused indefinitely. You
can buy silica gel in bulk. Take a small can with a compression lid
(small enough to fit in the conduit.) Poke a jillion small holes in it
(smaller than the granular desiccant or wrap with window screen material
with holes smaller than the silica gel) tie a string to it, fill it with
silica gel and insert it down into the conduit. Then seal the conduit.
If the seal works there is no problem. If the seal doesn't work, there
is no problem as the desiccant will absorb the moisture.
Since we all leave a string or rope run through the conduit in case we
want to pull another wire, use it to pull a towel through the conduit as
a moisture detection/removal measure. If it comes out wet or damp you
can then take action.
With Dryzit and similar products special containers must be used as the
dry chemical gets wet and becomes a liquid solution. I prefer silica
gel for a couple reasons. Not dissolving is one and you get it free in
packaging is another. Alternatives to a desiccant include blowing warm
dry air through the conduit. It takes very few cubic feet per min of
warm dry air to completely eliminate condensation problems. Of course
this requires you to not totally jam pack the conduit with cables and
leave a little space for air circulation. Unless you have a severe
condensation problem the warm air could be placed on a timer and only
run a small duty cycle for energy conservation.
Hot humid climates can have significant condensation problems and taking
preventative action is prudent.
Oh, about my cracked conduit due to my laziness and sloth re backfilling...
My wife was in the barn where this conduit terminates and came to tell
me water was coming out of it. I went and to my surprise there was
about 4-5 gal/min flowing out of the end of the conduit which is about 2
ft above the concrete floor. she kept placing 5 gal buckets under it to
catch most of the water. She would lug one to the door, dump it, and
return to find the other 5 gal pail nearly full. I realized this wasn't
a good solution so went back to the house for a cordless reciprocating
saw. Outside the barn where the conduit came up out of the ground at a
45 degree angle to penetrate the wall I cut a notch out of the conduit,
careful to avoid hitting any wires (sleeved in 1 inch irrigation
tubing.) This let water exit the conduit outside the barn, Yahoo
success. Next day... same problem. Water is gushing out the notch I
cut and overflowing the end of the conduit 2 ft above the barn floor. I
cut a larger notch at ground level and that permanently solved the
problem of water running into the barn.
I had previously detected the fact that the conduit was compromised by
noting the water on the pull rope (and rag) which I had run by using a
shop vac to blow a balled up rag through the conduit. So I still use
that conduit but cable bundles are sleeved in the irrigation tubing
which is 1 inch in diameter and good for 100 PSI (way beyond
condensation.) I recommend the flexible irrigation tubing to use as
sleeving on wire bundles in conduits where condensation or water ingress
could otherwise be a problem. Silicon caulk or similar could be used to
seal the entry/exit points where the wires enter and leave the
irrigation tubing for really strenuous cases.
Lowes has the irrigation tubing in 100 ft rolls for $12.
Patrick NJ5G
On 1/28/2016 7:22 AM, StellarCAT wrote:
My ongoing tower/antenna projects... hopefully I’m not boring others out
there...
I had planned on burying all lines to the towers (2, one at 150’ out and one at 300’ out) ... at first I
thought in solid joined (sealed) PVC which I can get for $9/10’ ... and I’ve also thought about direct
burial which for the coax and control cables has the added benefit of increased capacitance to ground and thus a
reduction of energy (lightening event) that reaches the shack... but control cables aren’t normally rated for
direct burial... or at least I don’t think so (researching that now)...
Then this latest QST article on coax says DON’T bury in any type of conduit be
it plastic or otherwise. It says condensation WILL get inside and ruin (flood) the
cables.
So .... if the cables are underground – below the frost line – say minimum 1’ down (upstate SC) ... how will condensation develop? Or
will it? I’m an engineer (HW/SW development) and reasonably intelligent but find it difficult to wrap my head around this issue... will it or
won’t it – flood that is? if it were always under ground – including both ends then I’d think not.... but it isn’t always
under as the ends come up to and above the surface... does that introduce a temperature variant enough to induce condensation? And I don’t like the
idea of holes in the plastic – that just means mud will indeed eventually work its way inside.
Maybe use corrugated tubing (again – no cuts or openings) ... this would allow
whatever moisture there might be, and I can’t imagine its going to be a great deal, to
settle in the lower points of the corrugation.
Comments from others that have been through this - in the south with similar
conditions would be appreciated.
Alternatively I could use a leader line and go from say 15’ on the first tower to the second tower and then that
same level to the shack with supporting 4x4 poles along the way... but this seems iffy as well as first the cables are
‘leaving’ the tower above ground potential so that might mean a higher voltage (common mode –
probably?) on the lines (again: lightening type event)... and the first tower is a rotating tower ... although thinking
about it that might work quite well to go from the tower with a ‘swing-arm’ of hanging coax/cables to allow
rotation.
thoughts?
Gary
K9RX
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