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Re: Rain affecting signals and sensing rain.


Hi

Thanks for your letter.

I have written to them. However, I am not sure if the sensor is heated
so that it will evaporate the moisture quicker. I think it would need
to be heated to give a more accurate reading of when the rain stopped.
If it is not heated the water will take a long time to evaporate and it
will still be registering rain when there is none.

Chris Jones mentioned that he has one with a heater. I would be
interested in a diagram of it when he can post it. I am not sure where
he got it or if he made it up.

So do you think these sensors will be able to tell you when there is
light drizzle and when it stops?

Many years ago I also heard of a sensor that had some sort of salt or
mineral coating so that when it was wet it would conduct electricity
better. This could overcome the problem that water has a very high
resistance and also make the sensor more accurate. However, a problem
could be that the salt or whatever is used may wash off. Do you know
any of them like this?

Your help is appreciated,
Regards Richard.


Re: Rain affecting signals and sensing rain.



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True - I think it's designed to model a leaf more than rainfall as such,
although you could retrofit a heater underneath it easily enough.

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Again, with a heater possibly, although there may be an error when it stops,
as it might take time for the water to evaporate. That said, the unit's
output will vary coninuously with the quantity of water on the surface, so
as soon as it stops I expect you'd see some change.

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This design is *very* sensitive - it responds to clean water.







Re: Rain affecting signals and sensing rain.


Hi

Thanks for your letter.

So is the sensor just a circuit board or does it have some chemical on
the surface to increase the conductivity of water?

I would also be interested in how the other ones mentioned by other
people compare to the sensor at http://www.emesystems.com/lwet_dat.htm

For example, Chris Jones said he would send some info. Does anyone know
his contact email address?

Your help is appreciated,
Regards Richard.


Re: Rain affecting signals and sensing rain.


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They do seem to have a painted version, but I don't think this is to improve
conductivity. As I mentioned, I've built the circuit for another application
(moisture sensing) and it is very sensitive even to clean rainwater.




Re: Rain affecting signals and sensing rain.



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Wasn't me... is this another Chris Jones?



Re: Rain affecting signals and sensing rain.


Hi

Sorry Chris. Actually it was Match who wrote the letter. It is just
that your name was near the top and quoted so I thought it was yours.
Do you know the contact details of Match, or could he email me the
information?

Your help is appreciated,
Regards Richard.


Re: Rain affecting signals and sensing rain.


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[...]

 What are you replying to?

GB

Re: Rain affecting signals and sensing rain.


There is another kind of senor no one has mentioned, that may be worth
considering...
VIBRATION / SOUND.
If you are only considering droplets moving largely under gravity, then a
tallish un-resonant column with a sound-absorbtive inner surface (to reduce
wind noise even more) may nicely complement ( a resistive sensor) , or even
suffice as a 'drizzle' detector. It could/would work by having a
large-surface area thin 'plate' connected directly to either a piezo or
speaker-type tranducer. Fed straight into a low-noise high gain audio amp,
the output would be super-amplifyed 'pings' of dirzzle landing on the
'rain-microphone'. The problem then of course is decoding this into amount
of drizzle. I suggest putting output straight into sound card of PC,  simple
bit of SW would be able to separate out wind noise from droplet pings,
provided some calibration work is done before hand.
An active bandpass filter after LNA would be wise too.

In your kind of problem, it may not be a simple matter of "drizzle or not?".
One might consider there to be a 2 dimensions of atmospheric water : droplet
size, and droplets / volume (density) , where drizzle free, high humidity
represents very small droplets, but a high density, and a shower in a dry
southern area might be large drops, but low density.
I only say this, because it is likely that for good accuracy, it is often
better to combine detections of 2 or more sensors (eg radar and resistance)
to increase probability of correct output. (EG With resistance detector, How
do know the difference between high humidity and drizzle? ..(without also
knowing humidity) .)

Another thing that has not been discussed is droplet resonance, where at a
particular radio or sound frequency, the absorbtion of energy by the droplet
rapidly increases.
There should be science literature (esp at BOM) available that discusses
droplet size for drizzle, and any decent sound or radar book will have a
small section (or refs at least) on radio / sound properties wrt drop-size.

If you are lucky, drizzle droplets will all be roughyly  the same size,
meaning that a simple ultrasonic path-loss measurment at a narrow band
centred on the resonant frequency will reveal drizzle.

Obviously a good topic, with so many responses!
Good luck.
David Merrett.

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Re: Rain affecting signals and sensing rain.


Hi

Thanks for your letter.

Sorry not to reply sooner.

We had a cyclone which you should have heard about on the news.

How difficult or expensive could this be to set up? Could one of the
options you said be transmitted over a couple of miles to determine if
there is any drizzle over that distance?

Your help is appreciated,
Regards Richard.

Simone Merrett wrote:
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Re: Rain affecting signals and sensing rain.


Richard,
If I were you, I would firstly  get some contacts at the BOM, and ask them
about this problem. They have probably thought about it before.
As a hobbyist, cost is an issue - so whatever you try will be partly driven
by your'e expertise with the technology... (the more basic the components,
the more work required to develop interfaces.)

I like youre IR tx-rx idea the best, and I doubt you'll find many people
around the world who specifically tackled the problem.
An IR laser diode focussed to a narrow beam (or even just a laser pointer).
, and a di-chroic filter at the same wavelength for the RX (again, focussed)
so you dont get drowned out by ambient levels.

Talk to some optical people for this one.... research the technology
yourself, and try to copy a similar system (eg..lookup "LIDAR", as used for
ocean depth measurements), then perhaps contact physics dept.s or optics /
electronics suppliers for the parts. You'll need an amplifier of course, and
some basic electronics understanding, or someone willing to volunteer the
time to design it for you.
David merrett

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