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Re: pure water capacitor?

Well...

	It is a curious idea but not practical.

	Pure water has some interesting properties, one of which
is that it is a very poor conductor.  A friend of mine once worked
in a repair depot for the navy and there was a radar system that
used a pure water jacket as part of the shielding around the klystron
oscillator.  The women who worked in the area kept taping the
jacket for water for their soldering sponges.
	Management was not happy.
	Pure water, chemically pure water isn't easy to produce,
and it is expensive to purchase and keep pure.
	The problem is that there is a certain percentage, a minute
percentage, of the molecules in water that spontaneously dissasociate.
The effect is that in some small portion water is always both acid
and base.  It is the universal solvent.  Given time and enough water
flow it will, quite litterally, dissolve anything at all.
	This property makes it very difficult to produce water of
the purity you speak of, and very hard to keep it pure since it will
attack whatever container it is  in.
	Admittedly, the dissolution is slight but it doesn't take
a whole lot to change the electrical characteristics of water.  It just
loves to support ions of any stripe.
	If you wanted to spend the money to produce or purchase
pure water, reagent grade or better at the very least and that's probably
not good enough...  and then you managed to create a tank that
would let it stay that way for a reasonable time, what happens
when you put the conductive plates in the tank.  Any good conductor
you put in the water would contaminate it almost instantly.
	You might get it to work for a time but I don't know for
how long.
	The other thing is that water molecules are electrical dipoles.
Microwave ovens heat your food by makeing water molecules dance.
So, you'd probably get a lot of energy loss into the water.
	I'm not surprised that water has a high dielectric constant.
It has a lot of mobile charged bodies that can swing around in response
to an impossed external field.  But high dielectric constant is just one
property to look for in a dielectric.  The loss factor is as important if
not more so.
	I expect your water capacitor would turn out to be a wonderful
tea kettle in actual use.
	And the water would be very very expensive...

	Maybe you could sell it to the ageing Yuppie crowd?

	I think you'll have much better luck doing something
more conventional, and save a lot of time and money too.

	Good luck.

	John

>I've been trying to do some research regarding design issues, when I came
>across a list of dielectrics for some materials for use in capacitors.  The
>dielectric constant for pure water is around 85 with an infinite breakdown
>voltage!  This means that two 12" x 12" plates spaced 1/8" apart will give
>a capacitance of .020uF!  I thought it would be a pretty sweet idea: a pure
>water cap in a plexiglass (or lexan (?)) box.  But, I have NEVER seen this
>mentioned anywhere I've looked.  The only problem I can see is
>contamination: everything has to be 110% clean before adding water.  This
>is do-able.
>
>Are there any reasons why I shouldn't try to use this?