Measurement of Electrical Discharge Machining Oil Quality by Analyzing Variations in the Equivalent Relative Permittivity of the Capacitive Sensor. - Shih-Jui Chen; Yi-Li Chen; Yu-Jui Chang; Dong-Lin Chuang; Yi-Chun Chen; Hai-Ping Tsui; Yean-Ren Hwang (Sensors, 2020)