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'[OT] Humidity sensors (Weird water !)'
1999\10\25@191027 by Darren Logan

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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

1999\10\25@193356 by Tony Nixon

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Only yesterday afternoon, I had to figure out a way of detecting a crack
in a steel tube that is welded to a base plate, and is constantly being
stressed in one second cycles. Eventually the tube cracks at the welded
joint, and then finally bends and breaks. The trick was to detect when
the crack appeared, which could take days.

The tube was filled with water above the level of the joint and I made a
moisture sensor out of placing two thin wires spaced about 1mm apart on
the sticky side of some cello-tape. I then stuck some paper towell on
the top of this and trimmed it so that I could wrap it around the welded
joint. The paper acts as an insulator until water seeps out of the
crack. The paper towell is very absorbent and as it quickly soaks up the
minute amount of escaping water, the resistance between the wires
changes and raises an alarm.

The things you have to come up with :-)

--
Best regards

Tony

http://www.picnpoke.com
Email spam_OUTsalesTakeThisOuTspampicnpoke.com

1999\10\26@014603 by w. v. ooijen / f. hanneman

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>
>     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.
>

The morale: use AC, without any DC component, dus as if it were an LCD.
Wouter.

1999\10\26@084739 by paulb

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Darren Logan wrote:

>  When water  droplets landed on the tile, the water acted to bridge
> the tracks and hence lower the resistance between the tracks.
> However, after the initial application of a water droplet, the
> resistance would quite rapidly increase (over minutes not hours).

 Remember that rainwater in a clean atmosphere is an insulator.

>  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.

 If it was rainwater, the only reason the droplet would conduct in the
first place would be contamination - of either the rainwater (or other
condensate) or the surface.  Very small amounts of contaminant would be
dissociated by the current and react with the plates, so that the
electrolyte would be removed.

>  Or maybe there was some sort of chemical build up of insulation
> around the base of the droplet... ??

 As someone else mentioned, "polarisation" may occur as gas bubbles
(oxygen or hydrogen) coat the surface.
--
 Cheers,
       Paul B.

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