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Cap Question
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