Soil Sensor: Does power type affects measured values?

Is it possible, that power type affects measured values of soil moisture? I am measuring soil moisture for few days. Very interesting is that after using battery power instead of USB, measured values are different. See the picture below. Since I started to use battery powering, the values are higher. It is no so logical, because I didn’t watered the plant - I expect, that soil mositure will go day by day down and not up. I don’t modify the application during these days, so it is not affected by any application change.
If voltage affects measuring, what is the correct value. And what about, when the voltage goes down - it is also affects the values?

Hi! Thanks a lot for reporting this.

For the moment being, we strongly suggest to power the Soil Sensor only from the battery source in order to minimize external ground capacitance.

The measurement should be voltage-independent, still, we plan to run more tests.

Today I looked into the PCB layout and with help from @Karel found some room for plane ground design, so it is more isolated from the capacitance measurement pins. Whether it helps is really difficult to tell, unless we test it on the real hardware (but it can only help, definitely will not cause any harm).

Also, @Kamil did a verification of the effect of the non-isolated power supply and we can say it matters a lot when the medium (water) is grounded and the Soil Sensor is powered from the mains.

TLDR; For now, use the Battery Module only as the ideally isolated power source for the Soil Sensor. It performs well under such circumstances. We will update you once we have the new PCB layout in our hands (should be a couple of weeks from now).

Thank you very much. P.

Here is the screenshot how the new PCB plane gorund design looks like:

And this is how it looked previously:

Thank you for your interest and quick response. I measure soil moisture in the planter (bucket) so it is not grounded. During March I will move it to greenhouse so then it will be grounded. If you want, I can move it to the greenhouse immediately for some test purposes. I don’t plan to use the sensor powered by USB in the real scenarios. But during development it is more effective for me. At this moment I know that it is not so good idea :slight_smile:
I am afraid, that I am not able to test if battery voltage affects measured values, because I am measuring real soil moisture and it is affected by more things. (air humidity, temperature, the plant itselfs)
I will be happy if I can help you to improve this sensor, don’t be hesitate to contact me for further tests, also for tests of new sensor.

If I understand correctly, you will perform some test to check if voltage affects measured values. (theoretically I can do similar test with discharged batteries…)

And two more questions to clarify how to use the sensor correctly:

  • sensor must be inserted into soil exactly until mark “100” (whole inserted into soil)?
  • sensor precision is 5% (step) and it is not possible to measure it more precisely?

Thank you!

does someone know if sensor really must be inserted exacly until mark 100 (whole inserted into soil )? I understand, when I insert it whole into soil, the measured value will be correct. But, what about when I insert just half os sensor into soil. Is it possible to recalculate value by this formula? (1/ [% of insertion] * measured value ) for example 1/0,5*25 => 50%.
I am not sure if it is possible to calculate it as simply as I wrote, or if the sensor must be inserted whole in the soil.
Thanks for any idea

Hi, not sure if that relation is linear. You would need to make your own calibration if you put just half of the Soil Sensor to the soil.