Application of fringe field capacitance sensor for the study of water permeation in organic coatings

Water permeation in organic coatings can be monitored by measuring the capacitance change in coatings. A new method of measuring water content in organic coatings using a miniature fringe field capacitance (FFC) sensor is described in this paper. The FFC sensor comprises two planar interdigitated electrodes on a dielectric substrate which is connected to a measuring circuit. Coating capacitance is measured by measuring the sensor capacitance when it is in contact with the surface of the coating. Sensor capacitance is theoretically calculated based on IDC capacitor theory for various sensor geometrical parameters at different water ingression levels in the coating. The computed sensor capacitance is validated with experimental values of five miniature sensors which are fabricated by lithography. The calibrated FFC sensor is used to monitor the water permeation profile in the coating, and the results are compared with that of electrochemical impedance spectroscopy technique. The technique provides scope for developing a portable, hand-held, and noninvasive technique to measure water permeation in organic coatings in the field.