Subject: secondary coatings,sealin
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Wed, 4 Oct 1995 23:24:00 GMT
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Quoting EDHARRIS-at-MPS.OHIO-STATE.EDU (Ed Harris)
> I'm still confused by the advantage of hermetic sealing,
> though. Perhaps Richard can provide some more insight. A guess
> is that the closeed secondary prevents motion of the slightly
> ionized gases to regions where they can become concentrated
> enough to cause a breakdown....?
The secondary does not HAVE to be hermetically sealed, it is just
a good idea. The coil form needs to be capped after winding
anyway to prevent internal arcing. If you are going to cap, you
might as well cap airtight. I have said in the past that one
small hole may be drilled into the bottom end plate of the coil
form to equalize the air pressure. But I also feel it is still a
good idea to plug the hole when the coil form is PVC. Reason?
There are (and will continue to be) lots of PVC pipe coil forms
used for Tesla secondaries. These coil forms will hold a few
grams of water in the porus plastic. If you are going to go
through the trouble of drying the coil form throughly, and then
sealing the outside of the form prior to winding on wire; you
should also take the proper steps to see that the coil form does
not reabsorb moisture through the INSIDE of the coil form. This
can be accomplished by either hermetic sealing (which prevents
water vapor from getting inside of the coil form) or sealing the
inside of the form with a coat of sealer. Either may be used, and
at least one of these two methods should be used when winding
wire on PVC.
Moisture reabsorbtion into well dried PVC coil forms can be
documented. I was storing my secondary coils in a damp basement
for many years. I began drying PVC plastics when I discovered the
ability of this material to retain water in the porus structure,
but I had a few coils that had been previously wound on PVC pipe
that had not been properly dried or sealed. I popped the end caps
off of these coils and weighed them on a laboritory scale, then
dried them thoroughly. A couple of coils wound on a four inch
diameter PVC plastic form weighed in three to four grams lighter
(average) after drying gently for a few days...
Now these coils were all sealed OVER the windings with
polyurethane, and a couple of them had been hermetically sealed
with end caps, but I popped these caps off to allow airflow
inside of the coil form during drying.
Things being what they are, I did not get back to these coils for
some time. They were not recapped, and they just sat around in
the damp basement for about a month... When I went back to them,
I happened to drop one back on the scale only to find that over
1/2 of the weight lost during drying had been regained. After
some additional experiments I concluded that some of the lost
weight was due to a final "curing" of the oil based polyurethane
sealer on the most recently completed coils. It can take a year
or more for a heavy polyurethane sealer coat to fully cure. The
rest of the lost weight, that is the portion that was regained
after sitting in a damp basement for a month, was almost surely
water. I redried the coils.
Drying these same coils again resulted in more weight loss. This
time I capped the coils right away while still warm from the heat
source, and I made sure that the end caps were glued down
airtight with epoxy (I had already learned what happens when
explosive and/or conductive vapors are trapped inside the coil).
I weighed the coils when the epoxy cured, and again after two
months storage in the damp basement. There was negligible change
in weight after prolonged storage.
Of course if you are winding on plexi tubing you don't have to
worry about this stuff. But, as long as people are still using
PVC for coil forms I am going to make recommendations based on my
experience with the material, and it is my experience that most
Tesla secondaries are wound on this plastic.
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12