Hi there,

    You may think this sounds over-the-top, but it is something to bear in
mind:

    Once upon a time... I worked with a particular water detector sensor,
which was   basically a small ceramic tile (size of a finger nail), coated
with near touching  tracks. When water  droplets landed on the tile, the
water acted to bridge the   tracks and hence lower the resistance between the
tracks.

    However, what I noticed was, after the initial application of a water
doplet,
    the resistance would quite rapidly increase (over minutes not hours).
    It started off at something like a few tens of kOhms, and ended up a few
hundred
    kOhms after a few minutes... even though the doplet remained the same
size.

    I'm no scientist, but I guess this was something to do with the current
passing
    through the droplet - doing 'something' to the conductivity of the water.
    Or maybe there was some sort of chemical build up of insulation around
the     base of the droplet... ??

    My point is: Dont expect to read a stable/fixed resistance value from the
same
    water over time. The resistance WILL increase as time goes on - perhaps
even
    to a point where your detection circuit thinks it is measuring an
apparant open   circuit !

    I suppose keeping the current and voltage as low as possible may help.
    Perhaps even continuously reversing the polarity of the excitation
voltage may     help?? who knows.

    Just thought I'd make you aware of such a phenomena, which could otherwise
    catch you out.

Regards,
Darren Logan BSc