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Plastic Pop Bottle SW Caps
Last night I did a few experiments with a soda pop bottle PSWC (Plastic Salt
Water Capacitor) . The wall of a 2l plastic coke bottle measured 0.010 +-
0.0005 inch wall thickness. With a foil wrapping the bottle had measured
2.4 nF, I fully expected to get one or two nF more with electrolyte
replacing the outer foil, but to my surprise the capacitance increased to
8.5 nF !! Mike you were right about my skill at the subtle art of
bottle-foiling !
Then to the dungeon:
The test article, a 2l coke bottle with saturated saline inside, set inside
a cut off gallon of fermented gatorade and salt. A dead ant was allowed to
remain in the electrolyte. #14 Cu wire was immersed in the interior and
exterior electrolyte.
The test stand, a 450VA, 15 kV NST with a 2cm fixed gap, and a large PVC
pipe cap as an insulated support for the victim.
TEST 1 : When the PSWC was clip leaded across the gap, the arc became a very
loud and satisfying roar. The internal electrolyte level was about 2 in
above the external level, and streamers were observed over the exterior of
the dielectric in a band between the inner and outer electrolyte levels.
Especially bright discharges were seen on the dielectric close to the outer
electrode wire, and the test was terminated.
TEST 2 : Extra dielectric was placed as a spacer between the outer electrode
and the bottle dielectric, and high voltage was reapplied. Corona and
surface streamers were seen in the band previously mentioned. After about
one minute, the PSWC shorted out very suddenly. A small dielectric puncture
was found several inch below the surface, under a fragment of the bottle
label.
TEST 3 : Another PSWC was prepared with a well-stripped 2l coke bottle.
This failed after about 3 minutes at the inner (upper) electrolyte level by
arcing thru to the outer electrode wire.
Conclusions : Everybody is right. Polyethylene Terpthalate is a great
dielectric. Soda pop bottle PSWCs will puncture and leak. Foil is not the
way to go.
Recommendations : Try again using a metal outer vessel for more symmetric
current distribution, probably with a spacer covering the bottom to prevent
dielectric stress concentration. Equalize inner and outer electrolyte
levels. Use an oil layer above the electrolyte. Cleanliness is next to
Godliness and Saint Tesla, get bottles 'squeaky clean' before use. Keep
electrodes clear of dielectric to allow current density to diffuse thru
electrolyte. Symmetric current diffusion and smooth, clean dielectric seem
to be the keys. No failures were attributed to the dead ant.
I am grateful to the folks who gave me the benefit of their experiences, and
solicit any further input.
> ----------snip
> From: Payne, Will E [SMTP:will.e.payne-at-lmco-dot-com]
> Sent: Monday, August 24, 1998 7:31 PM
>
> My recent query on use of plastic soda pop bottles for SW caps reaped two
> sets of responses. Some folks warned me about their bad experiences with
> soda pop bottle SW caps, which perforate, arc thru and dribble SW on the
> floor. However, Mike Foster is a fount of useful knowledge on sucessful
> use of PET dielectric bottles at very high DC voltages.
>
> I am inclined to experiment, perhaps even discover a cure for whatever
> ailments have plagued previous attempts. The list provided sevral
> stories of past failures which I take very seriously, so I will be
> especially cautious of AC heating, corona and hot spots attacking the
> dielectric. Mike is right, I probably could have made the foil tighter,
> so I will try again with liquid electrolyte inside and out, which may also
> help prevent corona due to sharp points. Did anyone who tried soda pop
> bottles relate their difficulties to this ?
>
> Although a firm beleiver in theory, I believe it must be tempered with
> practice. Thanks to Mike and to all who sent me their experiences with
> soda pop SW caps.
>
> <snip>
>
> > ----------
> > From: Michael Foster[SMTP:michael.foster-at-mailexcite-dot-com]
> > Sent: Monday, August 24, 1998 19:52
> > To: Payne, Will E
> > Subject: Max et cap
> >
> >
> > If your 2-liter Coke bottle only had a capacitance of a
> > little over 2nf, then the bottle walls must be thicker
> > than the mineral water bottles, or you are not as good
> > as I am at applying the foil. Pop bottles have to
> > withstand 6 atmospheres of pressure because of the
> > carbonation, so they might be thicker than mineral
> > water bottles. Why don't you try immersing the pop
> > bottle in a larger container of salt water and see the
> > difference between the foil wrap and the immersion
> > type? The pop bottles and mineral water bottles are
> > both made of the same stuff, polyethylene terphthalate.
> > PET has a dielectric constant of about 3.2. Both the
> > dielectric constant and the dielectric strength vary
> > a bit with the average molecular weight of the polymer
> > and with the stress of molding, or how far the plastic
> > is stretched when it is blown into the mold. Similar
> > effects are taken into consideration when rating
> > commercial metallized polyester caps as the film used
> > is the same material as the pop bottles. In this case,
> > the film is "biaxially oriented" meaning that it is
> > stretched in two directions while it is hot. This
> > increases both the mechanical and dielectric strength,
> > which are really the same thing. I find that it is
> > important to have the salt water at near saturation to
> > work really well. In fact, one of my favorite tech-
> > niques is to saturate the solution with both salt and
> > Epsom salts (magnesium sulfate.)
> >
> > I have not tried any of these capacitors in a Tesla
> > coil, so all of my theorizing is just a bunch of words
> > strung together until I do. All of my experiments have
> > been with DC, but the voltages have been very high in
> > some cases. Just to see what would happen, I made a
> > salt water cap which was two cells separated by PET
> > film only .00048 inch thick (12 microns.) And no,
> > that's not an extra zero. Anyway, I charged this up to
> > 25kv repeatedly with no trouble at all.
> >
> > The reason I bring up all this stuff about the stress
> > and orientation of PET, is that if you heat up a spot
> > on this plastic, it tends shrink there and get a little
> > thicker. I just thought that with the addition of
> > water cooling on both sides of the polyester, this
> > might constitute a "self-healing" dielectric.
> >
> > Have any of these people who have warned against trying
> > this actually used this type of capacitor in a Tesla
> > coil or are they just a bunch of hand-wringing
> > theorizers like me. I guess I'm just going to finally
> > break down and build a coil myself to test it out. And
> > by the way, I will cut apart some 2-liter Coke bottles
> > and measure the wall thickness with a micrometer, which
> > is how I know the thickness of the mineral water
> > bottles. They are all amazingly consistent.
> >
> > Yours truly,
> >
> > Michael S. Foster
> >
> <snip>
>
> Will
>
>