>The simplest capacitate water level sensor I ever made was a
>piece of TV twin lead and a NE602 mixer chip. I used the
>twinlead in the frequency as a capacitor in the oscillator
>and counted the frequency which is inversely proportional to
>the depth of the water in the vessel.

>It appears to me that the water effectively progressively shorts
>out the twin lead making it appear shorter. It does not work as
>a capacitor with the water increasing the capacitance because the
>frequency go up with the water level not down.

It sounds to me as though the twinlead is being used as a transmission line,
and the surface of the water acts as a moving ground plane. I would reckon
that the length of twinlead from the oscillator to the water surface is
probably a half wavelength at the oscillation frequency, rather than the
normal 1/4 wavelength, as the water will try and make a low impedance point,
being an effective groundplane, rather than a high impedance point.

It may be possible to tune the frequency range of the oscillator using some
L or C at the oscillator end of the twinlead. A transmission line looks like
an L or C depending which side of the resonant frequency it is operating, so
by doing the right sort of things it could be possible to pull the frequency
closer to where the length of twinlead is a quarter wavelength, which will
mean the oscillator is running at a lower frequency.

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