Darren Logan wrote:
>
> 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

Interesting!

I built a soil resistance measurement widget in the past (for a geology
group who were looking for signs of strain/shear in periglacial soils,
where frost heaving creates "Sorted stone circles"; what happens is the
Mica particles in the soil align up so that they're aligned along the
direction of shear, and as they act as a nice insulator, you can then
determine their presence, 2-dimensionally, with such a probe, so you
know shearing is going on - otherwise the Mica would be randomly
oriented.)  (See
http://www.last-word.com/lastword/answers/lwa370physical.html for info,
http://www.snowcrest.net/geography/parks/cells.htm for something about
this, also http://www.snowcrest.net/geography/parks/pattern.htm for a
picture, the last 2 both in California (!) of stone sorting, due to
frost heaving.  The ones in periglacial areas are even MORE interesting
IMO <G>  Looks like they're figuring Mima Mounds out, maybe?)

They wanted cheap & easy;  I used 4000-series analog switches, driven by
a 555 and an inverter, to make a low-current H-bridge, then measured the
resistance with a regular ohm-meter across top & bottom of the H-Bridge,
this prevents one-directional electrical ion movement which will
polarize and cause lots of bubbles to form on one elctrode, changing
your readings.  Had protective diodes in there, of course <G>  You saw
what happens when you don't use AC;  If you blow air past & defog,
probably not a problem, of course!

So, yes, if you don't use AC (real or simulated!), you get real
problems, water LOVES to electrolyse & it really messes you up.  Been
there, was harassed, sold the T-Shirt <G>

  Mark