Optimization and fabrication of planar interdigitated impedance sensors for highly resistive non-aqueous industrial fluids

There is considerable interest in using interdigitated arrays as sensors in both aqueous and non-aqueous fluids. However, highly resistive nonaqueous fluids present substantially different design constraints than conductive aqueous and polar non-aqueous solutions. Analytical equations for the performance of planar interdigitated devices in highly resistive non-aqueous industrial fluids were developed and used to design a series of prototypes. Theoretical expressions describing the cell constant, bulk and interfacial resistances and capacitances of interdigitated sensors are reported, leading to conclusions on appropriate empirical and modeling evaluation methods for design and optimization of interdigitated sensors for testing in highly resistive solutions. Electrochemical impedance spectroscopy (EIS) was used to compare prototype performance to analytical equations. In addition, the prototype devices were compared to solid parallel plate electrodes, which are typically used for EIS-based industrial fluid monitoring. Results show that performance of the interdigitated array prototypes is consistent with the design equations; however, the comparison of EIS results using the interdigitated prototypes and the solid parallel plate electrodes shows that the prototype design needs further improvement and optimization. Possible improvements to the design parameters and validation of theoretical expressions for interdigitated sensors are discussed, leading to fabrication of optimal interdigitated devices for use in highly resistive non-aqueous fluids. (c) 2006 Elsevier B.V. All rights reserved.