Comments to: A Novel Low-Cost Instrumentation System for Measuring the Water Content and Apparent Electrical Conductivity of Soils, Sensors, 15, 25546⁻25563

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.

Keywords: Ohm’s law; electrode admittance; permittivity measurement; self-balanced bridge; soil moisture and salinity.

Figure 1

Successive steps of conversion from raw sensor output, direct voltages $V_G$ and $V_C$ in the case of a self-balanced bridge or alternating current ${\mathit{i}}_x$ and voltage ${\mathit{v}}_{exc}$ for a method based on Ohm’s law, to quantities of interest, soil water content ${\theta }_v$ and its salinity ${\sigma }_{ion}$. Quantities G and C are, respectively, the capacitance and the conductance of the electrodes embedded in the soil, while ${\epsilon }_r$ and $\sigma$ are soil apparent electric permittivity and conductivity.

Figure 2

Schematic diagram of a self-balanced bridge. The current $i_x$ flowing through the admittance to be measured, $Y=G+\mathbf{j}C2\pi f$, under the voltage $v_{exc}$ of an on-board oscillator at frequency f, is balanced by the current $i_{eq}$ generated by the bridge. $i_{eq}$ is adjusted to $i_x$ owing to the direct voltages $V_G$ and $V_C$ provided by a feedback loop. At equilibrium, they are proportional to $i_x$ and therefore Y. Synchronous detectors and modulators permit the conversion between alternating and direct voltages using $v_{exc}$ as the alternating reference. Conductance $G_{eq}$ and capacitance $C_{eq}$ give bridge sensitivity and are fixed by passive components.

Figure 3

Time series over nine months of a three-channel sensor located in a remote catchment of the French Southern Alps—one point every 10 min. Minor interruptions due to power supply. Soil conductivity profile in $\mathsf{\mu }$S·cm${}^{-1}$.

Figure 4

Soil real permittivity profile (see Figure 3).

Figure 5

Two techniques in alternating current to measure an unknown admittance Y with a trans-impedance—or current-to-voltage converter—in the circuit input: one based on Ohm’s law (left hand part), the other based on the balanced bridge (right hand part). When the bridge is balanced, trans-impedance output v is zero (as $i_x\equiv -i_{eq}$). The automatic adjustment to reach this state, in form of a direct voltage $V_Y$, is used as the sensor signal. In the Ohm’s law case, v is one of the sensor signals, along with the excitation $v_{exc}$.