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Re: Radical Flat Cap design




>Date: Sun, 20 Oct 1996 13:13:51 -0400
>From: DavidF4797-at-aol-dot-com
>To: tesla-at-poodle.pupman-dot-com
>Subject: Radical Flat Cap design
>
Dave,
	Good post .... but,
>It has been recommended that "multiple" layers of dielectric be used in order
>to minimize the chance of  breakdown due to flaws in the dielectric.  While
>philosophically correct, this practice usually causes problem when
>implemented. 
[snip]  
>
>First, polyethylene sheets trap air bubbles between them, the more layers,
>the more air is trapped.  A 120 mil dielectric of twenty layers will trap ten
>times the air of a 120 mil dielectric composed of two layers.  There is
>therefore a trade off between reducing the likelihood of failure due to flaws
>by using multiple layers -and- using the minimum number of layers to reduce
>the total quantity of trapped air pockets (which can also cause failure due
>to heating effects). 
There is also a trade off involving the imperfections in the sheets of
polyethylene. While the 0.030" and 0.060" thick sheets purchased from
a plastic supplier are of much better quality than the film procured
at the local hardware store, they still have flaws in them. I have
blown holes in 3 of my 4 caps that I have built: a single layer 0.060"
cap and both  triple 0.030". I've not blown a hole in my remaining
dual 0.060" cap.
> I propose that two or three layers of thicker
>dielectric -two 60 mill sheets, for example- is sufficient to reduce the
>potential flaw line-up problem significantly while at the same time *greatly*
>reducing the amount of trapped air.
>
All of them were vacuum outgassed with an industrial vacuum pump to
below 10^-2 torr and then filled with new transformer oil.

[nip]
>I propose that this problem may be solved by interleaving thin absorbent
>paper sheets between each layer of dielectric to improve the wicking action
>of the oil and more completely and quickly displace the air pockets while at
>the same time having a negligible impact on the dielectric strength and
>dielectric constant of the assembly.  I implemented this approach on the
>first -and only- rebuild of the cap.  The result was a spectacular success.
> *All* off-gassing stopped within 12 hours and *all* air pockets were gone by
>24 hours.  
I've been working on this method for rolled caps. The kraft paper
allows me to roll the multiple thin layers of polyethylene without
them skewing.

	Regards,

	jim