On Thu, 16 Oct 2008 20:31:56 GMT, "Jerry"
wrote:


"Jeff Liebermann" wrote in message
.. .
On Thu, 16 Oct 2008 17:17:05 GMT, "Jerry"
wrote:

Will the taped end will not leak with 80 PSI air in the conduit?
Jerry KD6JDJ

Well, if it does leak, the sand above the duct tape will dribble down
into the 3/4" conduit creating a rather awkward clean up exercise.
That's why conduit runs usually terminate above the ground, not below.
The trick is to find the conduit end without blowing off the seal.

Hi Jeff
It may be of little interest to the OP, but the sand might have difficulty
leaking back into a conduit with air blowing out of it. In addition, the
air might leak from a small rupture below the oipen end of the conduit.
The OP may have no access to compressed air anyway.

Jerry KD6JDJ

Welcome to rec.radio.amateur.antenna.physics

Ok, let's do the math. Dry sand weighs 100 lb/cubic-foot. There's
about 1 ft of sand above the end of the 3/4" conduit. The weight
action is roughly conical, so the volume of sand involved is (my
guess) about:
100 lbs/ft^3 * 1/3 = 33 lbs of sand.
All that it acting on a 3/4" diameter pipe, with a cross sectional
area of about:
Pi * 0.75" = 2.4 in^2
Therefore, the pressure exerted by the sand is:
33 lbs / 2.4 in^2 = 14 PSI
So, if he can pressurize the pipe to more than 14 PSI, he can lift the
column of sand sufficiently to keep it from dribbling into the pipe.

Of course, it's not that simple. Laminar air flow, pressure gradients
across the conduit, and the effects of the duct tape will ruin my
simplistic guesswork. Worse, the back pressure created by the
immovable column of sand will force some sand particles into the
conduit around the edges with the "reflected" air pressure. In the
middle of the conduit, the air flow is all out of the conduit, but
near the edges, it could easily be the other direction. I'm also
assuming that the sand is a perfect air seal, which it's not. To
prevent all this from happening, the minimum air pressure should be
about twice the 14 PSI, which is easily achievable with an air
compressor, but not a vacuum cleaner.

I'm also trying to imagine how the process will work. I see an air
compressor pumping madly away as the neighbors kids furiously dig
around the resultant sand volcano, as sand rapidly refills the conical
hole. In my never humble opinion, there's no way to prevent sand from
dropping into the conduit if the duct tape seal is broken before
excavating the end of the conduit.

Once the sand is in the conduit, just blowing air through the pipe
isn't going to magically elevate the sand particles 1ft or more in the
air. The air does not have sufficient mass to convey enough momentum
to move the sand particles, much less eject them vertically. For
that, one needs a denser medium, such as water. Shoving a plunger
through might work, but it's equally likely to jam sand particles into
the PVC conduit walls.




--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558