High $Q$ Microwave Microfluidic Sensor Using a Central Gap Ring Resonator

In this article, a highly sensitive and compact microwave sensor with quality factor (Q ≈3000) is proposed for identifying and sensing liquid samples using a central gap ring resonator (CGRR). The high Q is a result of surface current redistribution. Our sensor is designed to operate at 2.5 GHz with an active gap region, where the sample flows, in which the electric field is at its maximum amplitude. The sensor was simulated, fabricated, and tested for a new application to identify the condition of oil used as a coolant and an insulating material in high-voltage transformers. The efficiency of oil in this application depends on its insulation capability and water content. Since both of these factors correlated with the oil's dielectric constant, the CGRR sensor showed very good sensitivity for detecting the ratio of water in oil from the measurements of Q change (and its link to the oil's breakdown voltage) involving fresh and used oil samples.