[Prev][Next][Index][Thread]

Re. T-minus 7 days and counting...

Hi Kent:

Comments interspersed:

>Original Poster: "Kent & Kim Schaffer" <santoken-at-bright-dot-net> 

>Well I made a test saltwater cap today.  Here's the spec's:
>Container:    16oz Nestea Bottle (Sweetened-No Lemon)  empty of course!
>                    -Glass appears to be in the .050" thick
>Wrapped the bottom third (including the bottom) of bottle with Reynolds
>Heavy duty foil.

I would think you'd want at least the lower three-fourths wrapped in foil.
You want to get as much plate area as possible to get the most capacitance
per bottle.

>Filled with 12 oz. Saltwater
>                    -Mixture is 3 tablespoons salt to 12 oz water
>                    -the apex of the water is at the same height as the
>aluminum foil on outside of bottle.
>Filled remaining space to the top with NAPA brand 10w30 motor oil (I own a
>NAPA auto parts store).

Motor oil will work, but when the time comes to dispose of a mixture of
motor oil and water, well, I can't think of any environmentally ethical way.
I'd recommend laxative-grade mineral oil instead.  Definately flushable!

>Used 5/16" "all-thread" rod it's about 1" from bottom of bottle
>  
>Capped with an all rubber automotive type expansion plug.  This works great
>because it can be sealed tight and removed again.

I have no experience with SW caps, but having built a rolled poly cap in
a PVC enclosure, WITHOUT any provisions for expansion or pressure relief,
and having it violently explode, spewing oil all over my garage, I would
reconsider having a tight seal.

>Here are more questions for you to ponder:
>  
>1    Does anyone have a formula for calculating the capacitance of a
>saltwater cap based on physical properties?

Sorry, I don't.

>2.    We live in a rural area in Ohio.  We have a water well and the water
>is high in iron.  Is this a plus or minus for these caps?

It would make no difference.

>3    In saltwater caps, I would assume the saltwater is the storage for the
>charge.  Would it be a benefit to use as large (or small) as bolt as
>possible?  Or not important?

The salt water is just a substitute for a conductive coating on the
inside surfaces of the bottles, and a connection between that surface and
your bolt terminal.  The actual charge is stored in the glass dielectric.
The salt water itself stores nothing.  The bolt connects to the
conductive water to your wires.  You want as much surface area on the
bolts as possible in contact with the water.  Ideally you would fill the
bottles with mercury, but that would get just a bit hazardous!

>4    These rubber expansion plugs use a large rubber grommet that is
>squeezed between two steel washers.  The washer on the top (outside) of the
>bottle is actuall touching the glass (nothing else except for the rubber).
>Does this pose a problem?  I really can't see how it's just that the plans
>I saw called for a plastic lid to be screwed on the bottle and I couldn't
>find a cap that would fit this bottle.

No electrical problem, except for the potential explosion problem
mentioned above.

>5    The plans called for wrapping the exposed foil on the outside of the
>cap with at least two layers of electrical tape.  Why is this when if it
>would get next to another bottle this would just be the ground anyway?
>Wouldn't it be better if all the bottles touched anyway?

I suspect that the goal of the tape was to get the foil in as intimate
contact with the glass as possible.  Yes, the foil on all bottles must
touch, and there should be a foil or metal sheet that all of the bottles
sit on top of, and this connects all their outer plates together.  Or, if
the outside is not completely tape-covered, putting a BIG rubber band or
several wraps of electrical tape around ALL of the bottles to keep them
all in mutual contact will work.  This provides a much better connection
than relying upon the relatively poor conductivity of just salt water if
they weren't touching.

One last thought.  I don't know what the voltage rating of a single .05"
thickness of glass is.  Hopefully someone else with more experience can
add to this, but it may be that you'll have to build two identical,
complete sets of bottles, each in their own tray, and wire the two sets
in series, to have the resulting series cap withstand the 10KV of your
power supply.  Did your plans state how much voltage it could stand?

Regards, Gary Lau
Waltham, MA USA