Re: Sensor design - dielectric constant of water

"Klaus Kragelund" <klauskvik@xxxxxxxxxxx> wrote in message
news:1127220973.032040.45320@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Hi
>
> I'm working on a water sensor (simply detect water or no water). The
> dielectric constant of water is about 80. But what about the value of
> the constant with different pureties of the water (salt, pH)?
>
> The application is a pump which has a metallic surface connected to the
> earth wire. The sensor consists of a metal plate housed in a plastic
> enclousure mounted on the chassis of the pump. The capacitance measured
> is the one from the metal plate to the chassis - growing in value when
> the water is present.
>
> Right now it looks to be a capacitive sensor design. But I also might
> opt for a conductivity measurement of the water instead. Any comments
> and experience in this field?
>
> Regards
>
> Klaus

You don't need to reinvent this - its all been done before for low cost
water level sensing.

Use a metal pipe standing vertically in the water.

Down the center of the pipe you run a teflon insulated wire which goes down
the bottom, round a rod and back to the top, so that only insulation is
under the water and you don't have to seal the ends. Between the wire and
the pipe you have a nice capacitor with zero DC leakage. Capacitance is
pretty well proportional to the water depth and is insensitive to water
salinity and wire positioning. The pipe protects the whole thing nicely.

The secret is that the water acts mostly as an AC short and the teflon is
the dielectric.

Don't use PVC wire - eventually the PVC leaks.

To measure the capacitance, you use a 7555 timer with the wet capacitor in
the timing circuit. The pipe is the earthy side of the capacitor. The 7555
is stable with temperature. The logic output of the 7555 runs to your
microprocessor.

Google will find you schematics, pictures of this sort of thing.

Roger Lascelles


.

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