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If water has such a good dielectric constant why don't they use it?

Discussion in 'Electronic Basics' started by [email protected], Nov 3, 2005.

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  1. Guest

    I was looking at a dielectric chart the other day and notices that
    water has a rather high constant. Yet I have never heard about or seen
    any water capacitors. Any comments would be greatly appreciated.
  2. Bob Eldred

    Bob Eldred Guest

    Water was used as the dielectric for some inertial confinement fusion
    research capacitors. The problem with water is keeping it pure enough to not
    be a mild conductor. Pure water is an insulator but, water being a polar
    solvent, picks up ions from many things it comes in contact with. These ions
    cause it to conduct electricity to varying degrees. It's difficult to keep
    the water pure.

    It takes more than a high dielectric constant to make a good capacitor.
    Conductivity is a show stopper as are nonlinearites of the dielectric
    constant with voltage stress.
  3. John  Larkin

    John Larkin Guest

    The Er of water drops rapidly with frequency, and leakage/ionic
    contamination is a problem.

    Right, never seen a watercap.

  4. Don Bruder

    Don Bruder Guest

    Because water is "too friendly" - **PURE** water is an excellent
    insulator. But even a trace of impurities (dissolved iron or copper,
    thousands of different chlorides, peroxides, and hydroxides, various
    acids, and a huge list of other things) is enough to turn water into a
    conductor whose quality ranges from poor to excellent.

    So the answer to your question basically comes down to "they don't use
    it in caps because although they can *MAKE* it pure enough (through
    distillation or by burning hydrogen gas) they just can't *KEEP* it pure
    enough for it to be useful for more than a few seconds at a time."

    Some tesla coilers use banks of tinfoil-wrapped, beer bottles filled
    with a saturated salt solution as capacitors, but they're using the
    water specifically as a conductor - making up one plate of the capacitor
    formed by sandwiching the bottle glass between two conductors, with the
    foil being the other plate, but that's hardly taking advantage of
    water's *INSULATING* properties :)

    Of course, there's also another issue: Hydrolysis. Put voltage through
    water, and you get water breaking down into its component H2 + O gasses
    - Which means that the thing has a sharply finite lifespan - It's only
    going to act as a cap of the specified value for as long as there's
    still the proper thickness of water between the plates. Before too long,
    there will be no water (And the value of the capacitor will have changed
    to that of an cap using air dielectric - Significantly lower), since all
    of it will have decomposed into its constituent hydrogen and oxygen
    atoms through electrolysis. By that time, unless it's been vented,
    there's a third hazard: it's a stochiometrically perfect mix of hydrogen
    and oxygen gases, possibly under considerable pressure, just waiting for
    any spark to come along and turn it into a rather nice explosion. Since
    you're looking for a high dielectric value material to build the cap
    with, it seems likely that you're also looking at a cap that's going to
    be prone to flashing over due to the high voltages it's seeing. The
    first time it does once a gas pocket builds up, it's real likely to go
    "boom" in a fairly spectacular fashion.
  5. Tim Williams

    Tim Williams Guest

    Not only that, but pH = 7 and "16.7Mohm" (presumably 16.7Mohm-meter is
    meant) are two salient figures that limit you. The first, pH, is defined as
    the negative logarithmic percentage of protons (H+/hydronium H3O+) in the
    solution. 10^-7 (percent or factor-of-total, I forget) is the degree to
    which pure H2O ionizes itself, that is, the equilibrium of molecular to
    ionic components: H2O <--> H+ + OH-. (Since the reaction produces equal
    parts H+ and OH-, the pOH is also 7.)

    And if you remember your chemistry, acidic solutions have more protons (a
    low pH, which being a negative log means more protonation) while basic
    solutions tend to release more hydroxyl (OH-) ions (high pH, low pOH).

    The other figure, "16.7M" (I think that's right) refers to the resistivity
    of pure, deionized water. The conductivity comes from the small fraction of
    H+ and OH- charge carriers naturally present in all water.

    An easier way to make water resistive is to freeze it, trapping all ions
    immobile in a crystalline lattice. Without ions free to move, charge cannot
    move and it cannot conduct. I think the dielectric constant of ice is still
    high, so besides water's quirky expansion, ice capacitors may be useful. At
    least in Alaska, and Wisconsin in the winter. ;)

  6. John - KD5YI

    John - KD5YI Guest

    Do a Google search using "water capacitor" to see 646 hits. Here is one
    example of Panasonic using them in a digital receiver (model SA-XR55):

    "A Pure Water Capacitor containing a rayon separator is used as the
    electrolytic capacitor for each of the seven audio channels. With its high
    water content this capacitor offers excellent electrical characteristics and
    low impedance. The clean electric current is delivers to the speakers helps
    acheive a clear, transparent sound."

    Maybe it would be cleaner sound if they put soap in it. (snicker)

    I saw references to lasers using water capacitors and there were mentions of
    saltwater capacitors for use with Tesla coils.

    Lots of stuff to read. Enjoy.

  7. Jon Danniken

    Jon Danniken Guest

    Tesla himself used saltwater bottle caps, but the saltwater was only used as
    the conductor (plate) to the glass, the glass being the dielectric.

  8. .

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