Could be using TDR ( time domain reflectometry )
example:   http://www.tscm.com/tdr.html
water causes short

POD

On 2006-Apr 09, at 9:49 AM, Danny Sauer wrote:

Bob wrote regarding '[OT]: Fountain Project- Help Me Pick A Sensor?'  
on Sun, Apr 09 at 04:12:
> can't discipline myself to keep it filled. So... I am gonna design a
> slick way to sense when the water level is low so it can kick on a
> solenoid and pump some back into it.

The ultrasonic ("cold mist") humidifiers I've had all use a single
piece of wire that senses water.  I think similar things are used for
detecting leaks in places like server rooms and basements.

Kinda like what these people make and sell:
http://www.rletech.com/products/waterdetection.html

That might be something to further research - maybe just how those
things work... :)

--Danny
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