Abstract
We have developed a permittivity-based sensor using a new and more accurate measurement technique. We now investigate the optimum geometry of sensor electrodes to apply models of conversion of medium permittivity into its water content (and salinity), while accounting for field constraints of compactness and simplicity. Models assume medium volume sampled by sensor to be large enough to consider a quasi-uniform permittivity. Using this hypothesis, and verifying ex-post its consistency, we carried out an exhaustive study of sample volume dependence on electrode geometry. In particular, we examine the role of electrode diameter, oversight by previous works. Besides, our approach permits a direct experimental validation with profiles of sensor sensitivity. For a two-rod design with electrode spacing D and diameter Φ, sample-volume size and localisation are determined by the ratio α = Φ∕D. For α lower than 0.25 or thin electrodes – a geometry extensively studied and commonly encountered – theory and measurements show that the volume is concentrated around the electrode surface. As α is increased it becomes localised between electrodes with a more uniform sensitivity, like for a parallel-plate capacitor. Volume size at fixed D is also assessed, with its highest value between α = 0.30 and 0.50. We adopted this range for our sensors.
Acknowledgements
We are grateful to A. Heitz for his contribution to the experimental work during its student internship. The IPGP contribution number: 3985.
Disclosure statement
No potential conflict of interest was reported by the authors.