SYSTEM, DEVICE, AND METHOD FOR MEASUREMENT OF ION CONCENTRATION OF FLUID TO BE MEASURED

PROBLEM TO BE SOLVED: To provide a method for measurement that achieves self-calibration and self-compensation of a variance in a pump current value due to a manufacture tolerance of an oxygen sensor or an NOx sensor.SOLUTION: A measurement sensor 202 includes a pump cell 204 and a sensor cell 206. Efficiency and precision of measurement of ion concentration of a fluid to be measured are maximized by changing a flow of ions in a pump cell based upon the output signal of the sensor cell. A pump current flowing in the pump cell is switched between a positive constant current and a negative constant current when reaching upper and lower thresholds of the output signal. A pulse width ratio of a rectangular wave generated as the current is changed is compared with a pulse width ratio function derived from calibration processing to determine the ion concentration of the fluid to be measured.