Institute of Electronic Engineering, Chung Yuan Christian University, Chung-Li, Taiwan 320, China;
Graduate School of Electronic Engineering, National Yunlin University of Science and Technology, Douliou, Taiwan 640, China;
Department of Electrical Engineering, National Chi Nan University, Nantou, Taiwan 545, China;
drift phenomenon; long-term measurement; drift calibration method; drift calibration device; SnO2 pH electrode;