Presentation of a Complex Permittivity-Meter with Applications for Sensing the Moisture and Salinity of a Porous Media

Abstract

This paper describes a sensor dedicated to measuring the vertical profile of the complex permittivity and the temperature of any medium in which sensor electrodes are inserted. Potential applications are the estimate of the humidity and salinity in a porous medium, such as a soil. It consists of vertically-stacked capacitors along two conductive parallel cylinders of 5 cm in diameter and at a 10-cm distance to scan a significant volume of the medium (∼1 L). It measures their admittances owing to a self-balanced impedance bridge operating at a frequency in the range of 1-20 MHz, possibly 30 MHz. Thanks to accurate design and electronic circuit theory-based modeling, the determination of the admittances takes into account all distortions due to lead and bridge electromagnetic effects inside the sensor when working at high frequencies. Calibration procedures and uncertainties are presented. The article also describes developments to make the present sensor autonomous on digital acquisition, basic data treatment and energy, as well as able to transfer stored data by a radio link. These steps in progress are prerequisites for a wireless network of sensors.

Autonomous Sensors for Measuring Continuously the Moisture and Salinity of a Porous Medium

“…Previous articles only described a calibration process using reference capacitors $C_{ref}$ and resistors $R_{ref}$ placed between electrodes [11,23]. To determine finely the coefficients introduced above, in particular the residual inductance $L_S$ at fixed inductance $L_{el}$, a calibration with air and with water at different concentrations of KCl salt is necessary.…”

“…The relation assumes no fringing effect. They are either eliminated owing to a guard at each end, like in the case of multi-channel sensors [11], or taken into account by a correcting factor close to 1. The factor depends only on electrode dimensions, namely, the ratios $h/D$ and $\alpha =\mathrm{sans-serif\Phi }/D$.…”

“…Permittivity-based instruments produce in the medium under study electromagnetic fields propagating along a direction z , which corresponds to the axis of sensor electrodes or guide. Fields are solutions of Maxwell’s electromagnetic equations (see Introduction in [11,16]) and present the general form:$A\left(\mathrm{boldr}\right)ph(2\pi z/\lambda -2\pi ft),$ where A is the field amplitude at point $\mathbf{r}$ in the medium, $ph$ the field phase and $\lambda$ their electromagnetic wavelength. Both A and $\lambda$ depend on ${\epsilon }_r$.…”

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

“…They are the object of this comment. They deal with the correct designation of the capacitance sensor that the authors Rêgo Segundo et al develop relative to our own work [ 3 , 4 , 5 ]: the influence of sensor operating frequencies on sensor intended output, and that of the temperature.…”

“…In the next section, for a better overall presentation of the permittivity-based sensors, we will briefly review and classify the various techniques used in available instruments. We then elaborate on the fundamental differences between the two techniques, schematically shown in Figure 1 , Ohm’s law-based techniques and actual self-balanced bridge such as the one used in our sensors [ 4 , 5 ]. The technique is derived from the bridge method but modified to be suited for fast and repeated measurements in the field.…”