Wireless logging system?

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I'm after a some kind of wireless logging system for a property where a
number of moisture sensors are located around a central PC that logs the
readings. The moisture sensors are in clusters of 3 or 4, once a day a micro
(PIC) takes a reading off each sensor and transmits the cluster ID and the
set of readings. There would be about 6 or so of these clusters all within
about a 100m radius of the logging computer.

While I'm guessing I could use a UHF TX module hanging off the micro and
bodge up something on the recieving end, I wanted to check whether there was
some kind of system already available that would reduce the amount of code
and development to get it going.

Any suggestions?




Re: Wireless logging system?


There was a Circuit Cellar Design contest entry which does almost exactly
what you describe.

Check out http://www.circuitcellar.com/fi2003/F2002.htm

Lionel...


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micro
was



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Thanks Lionel - you're right, that's an excellent design to work off.

The only point of difference is the sensing - my application has a cluster
of sensors buried a different depths (30cm, 1.5m and 2.5m for example) to
monitor the movement of water from the sub-surface, down through the
rootzone and below the rootzone. That said, converting it to work with the
different sensor would be trivial.



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Out of interest what are you monitoring -  a vineyard?
Do you use it to save on water useage or to optimise soil moisture for
growth / fruit or whatever?
regards
rob



Re: Wireless logging system?



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In this application it's apple trees. Water wastage is not a great concern
in this particular orchard - although that could change in the future.
Rather, the idea is to monitor the rate of percolation of water through the
ground to below the root zone - there's no point putting a bunch of water in
when the majority is ending up below root level, thus instead of one 8-hour
cycle it may be better to do 2 x 4 hours over 2 days etc, depending on the
soil and water uptake by the tree.

 This is also important when fertilising through the irrigation system as
you want the nutrient to hang around the root zone and get sucked up. Beyond
that, it would obviosly be useful to get an accurate picture of the moisture
levels accross the orchard in order to optimise water use.

If it does work out, I might try it out at a friend's vineyard to see if it
has a similar application there.









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When I was an apprentice i remember working in with the CSIRO to do a
similar thing. They were using a transmission line with RF to detect
moisture and conductivity. From memory, they used two stainless rods
seperated by teflon as the sensor. Might be worth looking into.

The other thing i saw was sensors that measured water flow or sap flow
by using a heater and 2 temp sensors. Very interesting stuff. I should
of stayed in that industry.

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If you poke around Google, you'll find there are a whole range of different
techniques for measuring soil moisture - the most accurate seems to be a
thing called a neutron probe - I hadn't seen the RF idea in my research, but
given the earlier-mentioned project's use of audio to determine moisture
levels, it seems there are a large number of valid approaches.

As I mentioned in another post, I am starting with simple gypsum blocks
containing bare wires and measuring the resistance between them using an AC
signal to avoid electrolysis. The advatage is that the sensors are very
cheap, although with limited lifetime buried in the soil, and the circuit to
measure the resistance is very simple (555 timer). The required accuracy and
resolution isn't terribly high, rather it is more an issue of trends in the
data








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AC won't stop electrolysis

stainless-steel wires will corrode much slower than copper

Bye.
   Jasen

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I guess I'll have to try and track down some twin-core insulated SS wire.
Does that exist?




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Trust me, the conductors will outlast the gypsum.


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But will the resistance change as the conductors  corrode?




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Keep the duty cycle low. Use AC but only energise the stainless steel wires
when you are taking a reading.

Should last ages.





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AFAIK unfortunately no, you'd have to make your own...

but you only really need the SS at the ends of the cable, joining stainless
to copper in a way that'll withstand the elements can be tricky,
one way woulld be to spot weld it and then seal it with "liquid electrical
tape" or similar but some of those grease-filled "squeeze" connectors
that the telecom guys use should be almost as good if slightly more bulky.

--

Bye.
   Jasen

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If you cannot find stainless steel (it is available for use in MIG [metal inert
gas]
welding machines) then use the next best thing, nichrome as used in heating
elements, it
is very corrosion resistant.
    Bye Roy



Re: Wireless logging system?


We do something quite similar with our water level and temperature wireless
short range telemetry system.  Could fairly simply be adapted to your
application.  How many do you want?

Regards
John
http://www.electrosense.com.au


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wireless

That water level system with the neat sensor arrangement, wireless link and
high measurement resolution is very impressive John.

In this situation I'm proposing to install up to 6 clusters of 3 or 4
sensors each buied at different depths. Each sensor, in the first instance,
will be a gypsum block (2 wires stuck in a lump of plaster) measured using a
555 that generates a frequency proportional to the resistance in the block.
A PIC will count the frequency of each sensor and then send the results to
the logging computer which will generate a web page showing the data both
numerically and graphically.

If the system works, the gypsum blocks will be replaced by Watermark sensors
that allegedly last up to 10 years, but cost US$30 each

At the moment, having looked at the project that Lionel posted earlier, I'm
leaning towards a transceiver based system where the logging computer polls
each client to report, thus avoiding collisions. I think I'll have a bit of
a play with the little UHF tranceiver module that Oatley flogs and do a bit
of testing - I have to use a micro for the moisture sensing, so it'd be good
if I can get it to do the comms as well.

That said, I might very well get jack of trying to get the comms bit to work
(which is why I was looking about for some kind of module).

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the
within
code



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work


Keep an eye out for an up coming silicon chip article (Dec or Jan?). A
little birdy tells me they are exploring the new RF transmitters from
Jaycar.



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