Gypsum Block Sensor

[weidong LU]

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Dear all,

I am a student from TUDelft,the Netherlands. My partner and I are doing a 
project which develops a system to water office plants automatically.The 
soil moisture required by the plants can be divided into three levels: High, 
Medium and Low. In this case, we need a soil moisture sensor to keep on 
monitoring the moisture level of the soil and decide whether to add water or 
not.

I are thinking of using Gypsum Block Senor, because it is cheap and meet our 
requirement(we don't need detailed value of soil moisture but certain ranges 
corresponding to three levels).

What's your suggestion about this selection of Gypsum Block Sensor? Do you 
have any better idea in how to implement this system? Please do not hesitate 
to let me know.

And I found that there were not so many Gypsum Block Sensors on the market. 
The famous one may be Watermark. Can I buy it in the netherland? If you know 
some companies in the netherlands or around sell Gypsum Block Sensor, please 
tell me as soon as possible.

Best regards,
weidong LU

===8<===========
[Bruce comments
Yes, some sort of electronic switch that is triggered to irrigate when
certain threshhold resistances values are met would seem a good way to go...
I'm sure the irrigaters in the list will suggest how to do this
in a practical sense?
Have a look at http://www.sowacs.com/sensors/gypsum.html as well as
the feature article http://www.sowacs.com/feature/mea/mea.html
]