In situ measurement of medium-frequency apparent permittivity using an electrostatic quadrupole. Application to the determination of the water content of wheat

Abstract

The objective of this work was to design, to build and to test a new instrument to measure in situ the water content of wheat, 1 or 2 months before harvesting, using a non-destructive and non-intrusive technique. We choose to measure dielectric permittivities, which are known to be correlated to the content of liquid water. Measurement is based on the electrostatic quadrupole principle: two charges of opposite sign are used to polarize the medium, while the resulting voltage between two other poles is measured. The size of the quadrupole is of the order of 40 cm. The static approximation applies in the low- and medium-frequency range. The transfer impedance of the quadrupole is inversely proportional to the apparent permittivity of the medium. Two synthetic models were developed: (i) a 1D model computing the quadrupole response in a stratified medium (air, crop layers and soil) and (ii) a 3D model computing the quadrupole response when a parallelepipedal body or a series of bodies is placed in the vicinity of the quadrupole. After several tests, in the laboratory and outdoors with a parallelepipedal body, the prototype was tested on actual crops. Measurements were performed in an experimental wheat field at the Grignon INRA experimental site. The variations of water content of ears and stems were studied versus space and time. A first law relating the water content and the apparent dielectric permittivity was established. The results show that this technique is relevant and that the water content of plants can be determined in situ, using this new non-destructive approach.