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Re: Blown Rolled "Hull" Capacitor Discussion

Hello all interested in the blown "Hull" capacitors.

After getting a few answers back on my previous "survey" of those that have
blown these caps, a trend is evident.

As I suspected, a vacuum was never pulled on those that have responded that
have blown the caps.  If you are going to all the trouble to make them and
have spent the money also, you should go to the trouble of using a vacuum
pump to remove all of the entrained air from the oil and the air that is
trapped between the plates.  I feel that this is a step that is greatly
neglected when building homebrew capacitors.  It is probably also the single
greatest cause of failure.  Air, above all else, is the enemy of capacitor
plates.

The second thing is the choice of oil.  Guys, transformer oil is only $25
per 5 gallon pail.  It is readily available.  The 2 most important features
of this oil (for our purposes) are its viscosity and its water content level.

The visocity of transformer oil is approx. ISO 7-9.  For those of you not
familiar with ISO codes, that makes it extremely thin and water-like.  That
means it will readily penetrate the capacitor's plates and bubbles will
escape fairly easily.  A vacuum pump should still be used.

Transformer oil is carefully made to contain an extremely low percentage of
water, less than .01%.  Believe it or not, regular oil found commonly contains
a fair amount of water, up to .5%.  The manufacturers say it does not contain
water, but my company has checked this out many times and found that this is
not always the case.  That is not good when you are building capacitors.
Transformer oil is also hygroscopic, water-absorbing.  That means that it will
"surround" any entrained water to help keep it from causing you problems.

Homebrew capacitors with 60 mill poly have served me well with 12 KV transformers.
There should be no reason that they fail at that voltage.  If you absolutely
can't get access to a vacuum pump, you should consider building a plate style.
A plate style can be assembled while in a tank of oil to keep air from getting
between the plates.  It is a messy process (one I got away from when I located a
vacuum pump I could use), but it works.  It doesn't remove the entrained air though.

I'm sure there will be people to respond that say they never pulled a vacuum and
their capacitors have worked fine.  I'm sure that is true.  Still, if you want to
make sure you have done everything to protect you investment, follow the 2 steps
listed above.  It is much better than playing the shoulda, woulda, coulda game
later on.  Not to even mention the time and money you are out.

Good luck,
Scott Myers