Visual inspection of the sensor after 3 weeks of use did not reveal any swelling or moisture expansion of the PCB material. Also, the paint on the sensor is still intact and did not wear out. However, while removing the sensor for inspection, I noticed that sensor readings are heavily affected by the immersion depth of the probe. Once pushed into the soil, you should never change it’s position!
Based on my experience with other battery driven TinyTX sensors (Temperature/ Humidity), I expect a battery life of about one year.
Thus, this setup seems to be suitable for long-term soil moisture measurements.
After two month of testing, I can conclude that this sensor type is very suitable for soil moisture measurements in contrast to the resistive sensors, which can be affected by corrosion. Its analog signal reproducibly reacts after watering as it is shown on the plot below. However, the signal/ peak height depends on the size of the flowerpot. It may take some time to spread the water by capillary force.
The next step is to program a shell script, which triggers an email notification, when moisture drops below 30%.
After 4 month of use, I took the sensor out of the soil for visual inspection. I couldn’t detect any signs of corrosion or swelling of the PCB material. I tested if the air- and water-values are still the same. There was no significant deviation (< 1 %) from the initial values. However, due to re-positioning of the probe within the flowerpot, the signal increased in the meantime to 90 % RH. After placing the probe to a different position, it returned to its initial value of about 60 % RH. This suggests that the sensor readings can be affected by other soil compounds than water or when the probe is placed in vicinity of plant roots.