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Re: detection of rain and close window
On 09 Jul 2005 11:32:34 GMT, andrew@xxxxxxxxxxxxxxxxxxxx (Andrew Gabriel)
wrote in message <42cfb5d2$0$38037$5a6aecb4@xxxxxxxxxxxxxxxxx>:
>In article <48snc1huuqn1vtdd019dm7lehapuu6h5i2@xxxxxxx>,
> MFHult@xxxxxxxxxxxxxxxxxxxxx writes:
>> A tipping bucket meter is the only commonly used device that I know of
that
>> "measure[s] quantity of rain in inches". A typical threshold is 0.04" or
>> so.
>
>I designed/built a very simple but effective rain sensor some
>35 years ago. It consisted of a piece of strip board (Vero board)
>with alternate tracks connected, and placed horizonally with the
>tracks uppermost, exposed to any rain, which would bridge the
>adjacent tracks and form a circuit. A separate circuit monitored
>for the bridging of the tracks via a simple 2 transistor amplifier
>driving a relay. It gave you a very early warning at the first few
>spots of rain, usually before you even noticed it, but no
>indication rate of rainfall.
>
>Some further refinement was a slight tilting of the board so rain
>would run off, and glueing about 10 resistors across the underside
>to provide something like 1W of heat in total which prevented dew
>formation and helped dry off the board quickly when the rain
>stopped.
>
>The final result worked perfectly for a number of years.
>Occasionally I had to clean off the top, although even when birds
>crapped on it, it still worked OK except for perhaps an initial
>false trigger at the time. (You could solder a few upstanding
>wires to the board to stop birds sitting on it.)
Neat! IC's with high input impedance have made this a bit simpler in the
intervening decades. Here's a current commercial example:
http://www.wintrol.com/rainsensor.htm
Note that Andrew would appear to have the fortune of living in the British
Isles where rainfall usually contains significant amounts of salt derived
from the ocean, and so has significant electrical conductivity.
(Conductance is the mathematical inverse of resistance.) Folks that live
in the center of large continents will find that rain detection using the
electrical conductance of the rain is not so easy because the concentration
of dissolved solids is typically less.
'Pure' water has extremely low electrical conductance -- about 10,000 times
less than sea water. Conductance is proportional to the concentration of
electrolytes ('salts') in the water, so on average, it is easier to make a
sensor trigger with scattered drops of rain in coastal areas than
mid-continent.
There are fixes, eg: pour salty water on sensor after each heavy rain and
allow to dry if the birds don't help out ;-) . However this will tend to
oxidize the surface layer of the sensing conductor grid, raising its
resistance -- hence the use of gold plating in commercially-made sensors.
HTH ... Marc
Marc_F_Hult
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