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Re: Cap Question

Aaack...
I'm not sure what would be happening, but one thought did occur to me. On
the Java
tesla calc page, they've supposedly got an error in their formula that
results in
it putting out 1/2 of the correct value. Maybe the people who did the programs
you're using took the already corrected formula, and multiplied the whole
thing by
2. Looking at the fact that 10.5 is one-half of 21, it looks like more than a
coincidence. I'd say, just build another cap like it, unless it's too
expensive,
in which case I'd say build some SW caps. I haven't heard about them having
problems, even with big coils.

Hope this helps!

Tesla List wrote:

> Original Poster: RWB355-at-aol-dot-com
>
> Hi All,
>
> Iīve got a really wierd problem. H E L P !!!!
> I decided to "oil in" one of my test caps. So I bought myself a piece of PP
> pipe, two end caps and proceeded on.
>
> The cap specs:
>
> Al foil: (2x) 78.74" long and 11.8" wide
> PE sheeting: (2x) 24 sheets of 1.75mil PE (42 mil per set)
>
> I started with 24 sheets of PE. Next is one AL foil. Then again 24 sheets of
> PE and stopped with the last AL foil. I used a wooden rod to roll the
thing up
> nice and tight. I didnīt have very much slippage (PE sheets) and it ended up
> rather nicely.
>
> Hereīs a bad ASCII Drawing:
>
> _________________ AL foil
> _________________ PE (24 sheets)
> _________________ AL foil
> _________________ PE (24 sheets)
>
> I put some tape and wire ties around it and removed the wooden handle. The
> electrical connection where placed in the middle of each AL foil sheet
(across
> the full width). I figured this would give me better discharge and
inductance
> values.
>
> Here the mystery starts.
>
> 1.) I had calcīd this cap to be aprox 21 nF. It turned out to having only
10.5
> nF. Thatīs only one half. I scratched my head, unwound the whole thing and
> tried to rewind it tighter. It didnīt help at all. Why? Is there a
possibility
> that the two AL sheets are acting capacitivly on each other, because I
didnīt
> cover the top AL foil with more PE sheets effectivly reducing the
expected 21
> nF to the measured 10.5nF ?!?. I donīt think so.
>
> 2.) I plunged it into my PP container and filled her to the rim with mineral
> oil. The capacitance didnīt change at all. I ran a line from the vacuum port
> to a glass full of oil. From the glass I went to my trusty vacuum pump
(an old
> fridge pump...works pretty good for the $10 it cost me). Next mystery. I
> pumped it down and measured the capacitance while pumping down. I figured it
> would go up as trapped air was being removed. Nope, it didnīt!! It actually
> went down aprox 2nF in the beginning. After 30 min of running I removed the
> vacuum pump and let the cap suck back the oil in the glass jar. There was
> still oil left after the vacuum was back to zero, so I figured it was
(almost)
> full. The cap now measured 10.8nF. !!#~-at--at--at-##+!! I reconnected the vacuum
pump
> at let her have it another 30 min. As the pump started making vacuum again,
> the cap value started to drop again, stopping at aprox. 10.2nF. I redid this
> twice more. I let the cap stand for two full days and measured it again. Now
> it has 10.95nF.
>
> 1.) Why doesnīt the cap have 21nF or at least somewhere near this?
> 2.) Why didnīt the cap value increase, as it was being filled with oil. I
> figured the PE and the oil should have a K of around 2 at least ?
> 3.) Why did the cap value drop as I removed the air ?
> 4.) Why didnīt the cap change itīs value (not much at least) after
standing a
> few days?
>
> Please help a poor coiler who is scratching his head in vain, chewing on his
> finger nails trying to figure out what he did wrong.
>
> BTW:
> 1.) I calcīd this value on my homebrew tc prog, but other progs gave me the
> same values, so it shouldnīt be a program bug.
> 2.) Yes, the vacuum pump really did pump out air. This was clearly
visible in
> the glass jar where the oil foamed and bubbled as air was being removed from
> the PP container.
> 3.) No, the PP container does not have an air leak. I greased the o-rings
> thoroughly before assembly. During the vacuum run I put a thin film of oil
> along the port and both end caps. No oil was sucked in and I didnīt hear any
> air being pulled in. So I figure it was vacuum tight.
>
> coiler greets from germany,
> Reinhard



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