Abstract
The article comments on claims made by Rêgo et al. about the sensor they developed to determine soil water content and its salinity via the admittance measurement of electrodes embedded in the soil. Their sensor is not based on a self-balanced bridge, as stated, but on a more common technique relying on Ohm’s law. A bridge is a zero method of measurement which can provide direct voltages proportional to soil permittivity and conductivity with a high resolution. Thanks to modern electronics the method can be adapted for fast and continuous monitoring in a remote site. Because of this confusion about the different measurement techniques among available admittance or capacitance sensors, we give a succinct review of them and indicate how they compare to the two techniques under discussion. We also question the ability of Rêgo et al.’s current sensor to determine both soil water content and salinity due first to instrument biases and then to the soil complexity as a dielectric medium. In particular, the choice of sensor frequencies is crucial in the two steps. In addition, the procedure to determine and account for temperature influences on readings is not presented clearly enough. It is important to distinguish between the effect resulting from electronics sensitivity, and those that are soil-specific. The comment does not invalidate the design of the sensor, but indicates points, especially parasitic contributions, which must be dealt with to avoid major errors.
Highlights
An operating sensor always presents an output
A sensor is mostly based on the determination of a convenient but intermediate quantity, here soil relative electric permittivity ε r and its conjugate soil bulk electric conductivity σ
We observe that in general the literature before 1990 did follow the correct definitions. Both measurement techniques rely on the contrast presented by soil apparent relative permittivity ε r between dry and wet soils, as do many other sensors
Summary
An operating sensor always presents an output. The question is how it is related to the intended result, here soil water content, θv , and water salinity, σion. Of equal importance in science and technique is the correct designation of measurement principles All of these fundamental considerations are the underlying points of the present comment. We observe that in general the literature before 1990 did follow the correct definitions (for instance [10,11]) Both measurement techniques rely on the contrast presented by soil apparent relative permittivity ε r (absolute dielectric permittivity normalized by vacuum permittivity ε 0 = 8.854 pF·m−1 ) between dry and wet soils, as do many other sensors. The procedure permits to discriminate between instrument biases and intrinsic electric effects of soil It is the two-step approach, summarized in Figure 1 in the case of capacitance sensors, admitted as the standard one [12]. Distinction between the Various Measurement Techniques of Permittivity-Based Sensors
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