Soil moisture evaluation with spiral fringing field capacitive sensors

This work reports the design, fabrication, implementation and testing of Archimedean spiral, fringing field, planar interdigital capacitive sensor employing printed circuit board technology to evaluate moisture contents in soil samples. Two sensor designs are adopted with differing numbers of turns, each in two different configurations (single sided and double sided), to evaluate their performance in the determination of moisture contents. Four different types of soil samples are subjected to experimentation in this study with either the sensors kept inserted fully into the soil samples or kept above a compacted soil surface. It is found that the sensor capacitance, in general, increases exponentially with soil moisture contents. The fitting curves are found to vary with water holding capacity of the soil. As the water holding capacity decreases, the exponential variation turns into a linear fit correspondingly. The sensor capacitance is compared with standard gravimetric method for soil moisture evaluation. It is found that the sensitivity of the present sensor is superior to all the sensors tested and reported in the past for soil moisture evaluation, with 437.48 pF/MC% for the double-sided spiral design and 229.73 pF/MC% for the single-sided design in alluvial soil. It also has other advantages like ease of implementation, high accuracy, fast response and low cost.