Screen printed and laminated electrodes for low-cost capacitive level measurement systems

The fabrication procedure and characterization of low-cost electrodes for capacitive level sensors realized on a flexible substrate are presented in this paper. The aim was to prepare conductive electrodes by printing of silver and PEDOT:PSS pastes on coated PET foil. Individual interdigital capacitors and a system with embedded microcontroller readout were designed for a comparative study. Individual capacitors in the form of interdigital electrodes (IDT) were designed with different finger width/spacing dimensions from 300/300 μm to 800/800 μm, a finger length 10 mm and 15 mm and an overall length of 100 mm. A demonstrator device featuring an integrated microcontroller, sensing and reference capacitive sensors and a resistive temperature sensor was realized to proof a practical utilization. The microcontroller is used to calculate capacitances of IDT electrodes in terms of charging time proportional to the fluid level. The design with reference capacitor can be directly applied to different fluids with a wide range of conductivities and dielectric constants without recalibration. The printed structures were thermally laminated with covering PET foil. The sensitivity of the fabricated devices was characterized in liquids with different relative permittivity and conductivity (water and oil). The highest measured sensitivity was 0.7 pF/mm and 0.08 pF/mm for water and oil respectively, with resolution down to 0.1 mm.