Sensitivity of capacitive throughput sensor to the change of material relative permittivity
Czech University of Life Sciences Prague, Faculty of Engineering, Department of Physics, Kamýcká 129, 165 21 Prague 6 Suchdol, Czech Republic; e-mail: jlev@tf.czu.cz
Abstract:
The capacitive throughput sensors have been tested in many applications (e.g. the throughput measurement of potatoes, sugar beet, chopped maize and hops). The results showed that the capacitive throughput sensors can be very perspective in some cases. The capacitive sensor for the throughput measurement can be described as a parallel plate capacitor where the dielectric is a mixture of air and the measured material. The equivalent dielectric constant increases with the increasing thickness of the material layer between the plates and the electric capacitance of the capacitor is increasing as well. The thickness of the material layer between the plates can be then determined via the electrical capacitance measurement. The main goal of this work is to describe the relationship between the relative permittivity of the material and the sensor output. The sensor values output directly depend on the sensor impedance and it is influenced by the electric field between the electrodes. The electric field is most influenced by the dielectric properties of the material and the distribution of the material. It was found that the influence of the relative permittivity change is significant only for less values (approximately 10 and less). These results mean that the material with the higher relative permittivity is useful for the capacitive throughput sensor. Also this behaviour can explain why the influence of the moisture is less significant for the moister material, because moister materials have higher relative permittivity.
Key words:
capacitive throughput sensor, moisture content., relative permittivity