Conductivity sensors

Hi,
I'm looking to measure when locations on the shore get splashed by
waves. I've tried lots of different things (direct voltage,
capacitance), but none have been ideal.

I've come across an interesting schematic, at:
http://www.emesystems.com/OL2mhos.htm

which uses a 555 timer to read the impedence across a probe. I've
breadboarded this up, and it works perfectly, measuring splashes of
water very quickly, but accurately detecting when the water has drained
away. I have connected the circuit ground to the black wire on the
schematic and am reading the voltage across the 1K resistor, which
gives a measure of the current consumed by the 555 (proportional to the
oscillation rate).

The problem is that I need to have several of these in close proximity
to each other. This means that when they are all splashed by a wave,
the probes will be shorted to each other. In practice, if I have 2 of
these probes running from the same power circuit, when they are both
immersed in the same solution, they act as if neither of them are
submerged.

I can't quite figure out what parts of the cirtuits are interacting
with each other. I have tried connecting the power to them through a
multiplexer, so they aren't on at exactly the same time, but this
doesn't seem to help.

Does anyone have any suggestions?

Thanks,
Mike

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