Comparative Investigation of Capacitive Fringing Field Sensors for Humidity Measurement

Humidity sensing is a critical parameter in various applications. The commonly used capacitive humidity sensors in the literature have an interdigital planar electrode structure that may be treated as a fringing field capacitive (FFC) sensor. Several forms of FFC sensors are reported for humidity measurement, but a comparative investigation of their response characteristics is rarely found. Therefore, this work presents a comparative analysis of planar capacitive sensor electrodes, for developing inexpensive humidity sensors. Five different electrode structures, interdigitated (IDE), serpentine (SRE), meandered (MED), spiral (SPE), and circular cross (CCE), have been selected for comparative analysis. The design of electrodes has been done to obtain an equal effective sensing area keeping the track thickness, electrode gap, and sensing film fixed. The fringing field contribution for each electrode structure has been obtained through finite element analysis (FEA) in COMSOL. Then, five FFC humidity sensors are fabricated with polyethylene glycol (PEG) hydrophilic sensing film. The sensors are exposed to different humidity levels (8%RH–80%RH) to obtain their static and dynamic parameters using mixing flow and saturated salt solution (SSS) methods. The comparison has been made on fringing field capacitance, sensitivity, response/recovery time, and influence of oxygen. The sensors with IDE electrodes show the highest sensitivity with a lesser fringing field effect, whereas the circular cross electrode has the better recovery with the highest fringing effect contributing 60% of its total capacitance value. However, the choice of electrode structure for fabricating the sensor will depend on its specific applications and requirements.