Soil Moisture Sensor Update

Update 2011-09-05: I have created a new blog about electronics only – check it out here: Electronic measurements, NSLU2 and soil moisture sensor

My 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.

5 comments on “Soil Moisture Sensor Update

  1. -

    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.pdf

    I guess measuring the soil temperature (also while calibrating) and using it for compensation should be possible..

    Keep up the good work!
    Michael

  2. -

    @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.

  3. -

    Hey Thomas,

    I have also been working on a similar circuit (based on that from the emesystems link above).

    The reason for the variations with temperature is that the resistance of the sensor changes with temperature. The variance is 1% per degree F or 1.8% per degree C. I have found that making this correction that I am getting far more accurate results. It is very important though that the temperature reading is taken right next to the watermark sensor.

  4. -

    @Simon: Thank you for the information – very useful. Good luck with your circuit 😉

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