Sample volume of a capacitance moisture sensor in function of its geometry

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.

Keywords:

• Moisture sensor
• sample volume
• spatial distribution of sensor sensitivity
• electrode geometry
• electrostatic energy
• dimensionless study

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.

Additional information

Funding

Sensor development is supported in part by the grant ANR-11-EQPX-0011 of the Agence Nationale de la Recherche allocated to the program ‘Critex’ – Challenging equipment for the temporal and spacial exploration of the Critical Zone at the catchment scale – (20 March 2013). It won the calls to submit projects for the ‘2012 Facilities of Excellence’ competition named Equipex. The program Critex is a French national project, which groups 21 laboratories from more than 20 universities and four research organisations. It will allow the study of surface and underground water relating to the characteristics of the natural environment and surrounding human activities. It will thus contribute to a better management of water resources in varied contexts.