That was actually something like my original idea, but then I found the
Quantum chips which seemed to have the capacitance sensing in a neat
package.  How would you measure the capacitance between the plates?

cheers,
mike



At 01:59 AM 7/9/2004, you wrote:
> >I'm looking for some way to measure when a probe is in
> >or out of the water.
>...
> >A couple of people on this list suggested that capacitance
> >might be a better way to go, and I'm looking to try that route.
>
>You might try and manufacture your own dielectric measurement sensor. This
>is what is being used on the Huygens probe that is going to be set adrift
>from the Cassini space probe that has just gone into orbit around Saturn.
>
>The sensor consists of a number of thin plates (it looks like about 10 thou
>stainless steel) about 4 or 5 square inches, and formed up like the old air
>spaced tuning capacitors in radios. There is a bolt through each corner, and
>odd numbered plates connected to one bolt, even numbered plates to another
>bolt, making a capacitor. The whole thing was then dipped into a plastic
>like stuff to put an extremely thin insulating coating on all the surfaces,
>so there can be no electrical contact with the liquid.
>
>The presence of liquid between the plates will now change the capacitance,
>due to the different dielectric properties of the liquid. Different liquids
>will produce different levels of capacitance change.
>
>Not clear what you are trying to measure, but you may need to have a large
>enough gap between the plates to ensure that when the sensor comes out of
>the water, the plates are far enough apart that there is no water retained
>by capillary action.
>
>Have fun experimenting.
>
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