Microwave Sensor for Nondestructive, Volume-Independent Liquid Characterization

In this letter, we propose a microwave sensor for nondestructive, volume-independent liquid characterization over a 50 mL volume range. The proposed sensor is based on a 1.08-GHz slot-coupled cylindrical dielectric resonator and liquid-positioning jigs that control the degree of coupling. To ensure a stable resonance over a 50 mL volume range, we introduce a new criterion in which the resonance frequency monotonically shifts with respect to both the volume and permittivity of the liquid under test (LUT). The sensor was validated with ethanol solutions in different concentrations and volumes. The sensor demonstrates a resolution of approximately 5% ethanol concentration regardless of LUT volumes and a sensitivity of 1.04 MHz/% averaged at 25 mL.