WiEps: Measurement of dielectric property with commodity WiFi device – An application to ethanol/water mixture

WiFi signal has become accessible everywhere, providing high-speed data transmission experience. Besides the communication service, channel state information (CSI) of the WiFi signals is widely employed for numerous Internet of Things (IoT) applications recently. However, most of these applications are based on analysis of the microwave reflections caused by physical movement of the objective. In this paper, a novel contactless wireless sensing technique named WiEps is developed to measure the dielectric properties of the material. This technique exploits the transmission characteristics of the WiFi signals. In WiEps, the material under test is placed between the transmitter antenna and receiver antenna. A theoretical model is proposed to quantitatively describe the relationship between CSI data and dielectric properties of the material. The proposed technique is applied to measure the ethanol/water mixture. During the experiment, the phase and amplitude of the transmitted WiFi signals are extracted from the measured CSI data. The parameters of the theoretical model are calculated using measured data from the known materials. Then, WiEps is utilized to estimate the dielectric properties of different liquors. The estimated permittivities and conductivities show good agreement with the actual values, with the maximum accuracy of 97.3% and 99%, respectively. These results indicate that the proposed WiEps is effective in measuring the dielectric property. This technique is promising to open up a new direction for IoT application with ubiquitous WiFi signals, which can be applied to food grading, manufacturing process monitoring, security check, and biomedical engineering.