On Fri, 31 Jan 2003, cdb wrote:

*>On Fri, 31 Jan 2003 01:25:58 +0200, Peter L. Peres wrote:
*>My (probably wrong) calculations show that a 2mm wire in a 10mm i.d.
*>tube,
*>20 cm long, should give about 0.5 pF in air and 41 pF in water, with
*>the
*>capacitance changing linearly with water level. Neglecting the
*>influence
*>of the paint dielectric, which is supposed to be very thin. I used the
*>plate capacitor formula for this (not the one for cylindrical caps)
*>so the
*>numbers are slightly off.
*>
*>I tried this method in my rain detector, from a circuit published in
*>Circuit Cellar and Elektor sometime ago. Just couldn't get it to work,
*>the 'capacitor was to be the frequency determining component of the
*>astable. I also tried detecting a pulse duration difference between wet
*>and dry with a timer working out the wet threshold.

What kind of rain detector ? The kind with a funnel that leads to a bootle
where you measure what fell ? The level sensor I described above works
fine, the last time I used it I used a sensor ready made and I made the
electronics. There is nothing magical about the sensor. If you don't
believe how well it works quickly build one on perfboard and dip it into a
glass of water. That's how I tested it out first (I am very wary of great
claims of performance and check everything I have time to).

This method is better than the astable method because almost all salts or
whatevers dissolved in water change the conductivity a lot but only change
the dielectric constant a little (wrt. water/air rato). Rainwater is
relatively pure and probably does not conduct at all if you have
reasonably clean electrodes (which yours probably were not).

Peter

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads