Extending the Limits of a Capacitive Soil-Water-Content Measurement

This paper describes a new electrode structure that has been developed to measure the water content of artificial soil. Similar to a conventional setup, the water content is also extracted from the capacitive component of the admittance. Results show that in the existence of high water conductivity $(\sigma \geq 1.5\ \hbox{S/m})$, the conventional rod-shaped pairs of electrodes are no longer suited for accurate water-content measurements. This is due to the occurrence of physical effects, including skin effect, proximity effect, and parasitic inductances. To reduce these physical effects, a new electrode structure has been designed, built, and tested. It is shown that, with the desired signal frequency of 20 MHz, the physical problems have significantly been reduced and that, even with water conductivity of up to $\sigma = 2\ \hbox{S/m}$, reliable results can be obtained.