The invention calibration (Buffer, standards) can be non-use and self auto-calibration without operation of the administrator of the solution for calibration by self-diagnosis whether the fourth electrode pH sensor status, and at least the sensor when measuring the pH characteristics of the water using the and detecting, for detecting and transmitting to, and more particularly quality characteristics of using a four-electrode pH sensor on the pH measuring system of auto-calibration feature is provided to indicate the exact measured value to the feedback operation converts the measurement signal of the detector conversion unit, receiving the converted portion signal arithmetic control processor, a display unit for displaying and recording the measured values ​​calculated in the processor, an operation for the setting operation with the processor, for power supply and electrically isolated to control the power to the isolations, connected to the processor by an interface for transmitting an operation signal to the outside; Through the localization of product development, reduce dependence on foreign technology is to increase the export competitiveness effects of goods and technology via the world's first technology commercialization.