Can you tell us more about the effect of salinity on the results? Is there some kind of capacitive effect of the double layer of ions in the soil?
It would be great if you could point me to more information about this. I am interested in making high-precision soil moisture measurements.
Imagine if there was a perfect conductor around the sensor (for example, if the sensor was immersed in liquid gallium metal), it would connect the sensing capacitor plates to each other so that the only insulator between them would be a thin conformal coating on the circuit board.
These cheap capacitive sensors, built on 555 chips, typically operate at frequencies in the tens of kHz, which is too low to eliminate the influence of dissolved salts. It may be low enough to cause other problems such as dielectric absorption, which manifests itself as hysteresis.
Note that the sensor board is actually a capacitor in series with the soil equivalent circuit, one on each side. You can also use an unshielded electrode without any coating for direct connection, but the electrode will quickly dissolve into the soil.The application of an electric field will cause polarization in the soil + water environment. The complex permittivity is measured as a function of the applied electric field, so the polarization of the material always lags behind the applied electric field. As the frequency of the applied field increases into the higher MHz range, the imaginary part of the complex dielectric constant drops sharply as the dipole polarization no longer follows the high-frequency oscillations of the electric field.
Below ~500 MHz, the imaginary part of the dielectric constant is dominated by salinity and, as a consequence, conductivity. Above these frequencies, the dipole polarization will decrease significantly and the overall dielectric constant will depend on the water content.
Most commercial sensors solve this problem by using lower frequencies and using a calibration curve to account for soil properties and frequency.
Post time: Jan-25-2024