Two 3D-Printed Sensitive Cylindrical Sensors for Characterizing Organic Liquids

In this article, two highly sensitive sensors based on cylindrical cavities for measuring the complex permittivity of organic liquids are presented. To analyze the sensing performance, the two sensors are designed at the working frequency of around 20 GHz, where the relaxation frequencies of some common lossy liquids are located. For liquid sensing, a Teflon tube is designed at the sensing area. Based on full-wave simulations, the characterization principles are provided, and additionally, the sensing range for the complex permittivity of lossy liquids is analyzed. Next, by using selective laser melting (SLM) additive manufacturing technology, the two sensors are fabricated. However, due to manufacturing tolerance, there is a slight difference between the fabricated sensors and the simulated ones. Therefore, further simulations are performed for calibration of the complex permittivity characterization formulas. The two sensors are used to measure seven pure organic liquids and ten liquid mixtures, which are commonly used for industrial applications. The measurement procedure is simple and nondestructive. By comparing with literature data, the two sensors are validated to provide reliable results. The experimental validation also demonstrates that the proposed devices have good sensitivity to the complex permittivity of liquids. Their good performance is also validated by comparing with other sensors reported in the literature.