Jean-Pierre Frangi

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

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

The article describes a new field sensor to monitor continuously in situ moisture and salinity of a porous medium via measurements of its dielectric permittivity, conductivity and temperature. It intends to overcome difficulties and biases encountered with sensors based on the same sensitivity principle. Permittivity and conductivity are determined simultaneously by a self-balanced bridge, which measures directly the admittance of sensor electrodes in medium. All electric biases are reduced and their residuals taken into account by a physical model of the instrument, calibrated against reference fluids. Geometry electrode is optimized to obtain a well representative sample of the medium. The sensor also permits acquiring a large amount of data at high frequency (six points every hour, and even more) and to access it rapidly, even in real time, owing to autonomy capabilities and wireless communication. Ongoing developments intend to simplify and standardize present sensors. Results of field trials of prototypes in different environments are presented.

Validation of a Complex Permittivity Meter for Porous Media Operating Between 1 and 20 MHz

This paper describes the procedure to qualify an admittance meter as a permittivity meter and a conductivity meter σsensor through validation with liquids over a range of relative permittivity εr values from 1 to 80 and with operating frequencies f between 1 and 20 MHz. The sensor is a capacitor which consists of two parallel cylinders of 10-cm typical dimension. A circuit-based model of the sensor was previously calibrated against accurate electronic components. A discrepancy of 10% is found between the calibration with components and the validation with liquids. All potential errors have been carefully examined: parasitic impedances of the electronic circuit and leads, modifications of liquid permittivity εrliq due to temperature influence, water contamination or relaxation effects, and possible fringing effects with the help of numeric simulations. Forty percent of the discrepancy results from the finite dimensions of the liquid-filled recipient. The fringing effects due to insulating rings at each end of the capacitor seem to be discarded. The analysis of the uncertainty on εrsensor and σsensor shows a relative uncertainty of 3%-5% due in large part to the numerical acquisition of high frequency.

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

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.

Neural network implementation for a reversal procedure for water and dry matter estimation on plant leaves using selected LED wavelengths

An inversion method based on a neural network was used to estimate water and dry matter contents on plant leaves, from transmittance and reflectance measurements, using light emitting diodes (LEDs) at specific wavelengths in NIR and FIR. The preliminary results for the predicted water content by the neural network method showed a RMSE value of 0.0027 g/cm(2) and |σ| value of approximately 3.53%, computed on 127 plant leaf samples over 51 species. Dry matter estimation also was performed, which showed potential implementation after future improvements. We believe this inversion method could be implemented in a portable system based on any silicon platform with the capability to perform in situ measurements on plant tissue.