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Soil Moisture Sensor Update
Apr 16My Watermark soil moisture sensor and additional circuit has been running 24/7 for about 4 months now, and it really is a stable system. Here are the data collected so far:
On the week graph you will notice an oscillation peaking high at 6 PM and low at 10 AM every day:
Compare the above graph to the weekly temperature graph and you will see the connection to the soil temperature:
When the soil temperature goes up, so does the soil moisture reading. It could be that the accuracy of the soil moisture measurement is highly temperature dependent. Another explanation could be, that air is able to contain more moisture at higher temperatures and this more humid air somehow spreads to the soil and raises the soil moisture level.
What do you think is the explanation? Please leave a comment.
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3 Responses to “Soil Moisture Sensor Update”
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Michael said on May 7th, 2009 at 3:03 pm
Inspired by your site I got the same devices.
If you find anything further about the relation of soil-temperature and current readings I’d be very interesed..I found something very interesting here:
http://www.emesystems.com/pdfs/SMX.pdfI guess measuring the soil temperature (also while calibrating) and using it for compensation should be possible..
Keep up the good work!
Michael -
Marc MERLIN said on May 10th, 2010 at 6:22 pm
Yes, the humidity sensor is super temperature dependent.
See
http://www.mail-archive.com/owfs-developers@lists.sourceforge.net/msg06064.htmlMarc
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Thomas W. said on May 19th, 2010 at 10:25 am
@Michael and Marc:
I found this on Wikipedia:
“Gypsum is moderately water-soluble (~2.0 – 2.5 g/L at 25 °C) and, in contrast to most other salts, it exhibits a retrograde solubility, becoming less soluble at higher temperatures.”
AFAIK, the sensor contains a gypsum block.
This is strange, because the data shows that when the temperature goes up, the moisture level goes up too.
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