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Re: Cap Question

Reinhard,
My calculation came up with 9.8nF, near what you measured. There got to be
something wrong with your calculation:

    .224 * k * A / d * N

 = .224 * 2.0 * 929 / 2 * 2 / 1000000 = 9.8nF

k = dielectric constant (2.0)
A = plate area in square inches (78.74 * 11.8)
d = distance between plates (.00175 * 24)
N = Number of plates (2)
/ 1000000 = nF
Bart

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