Patent ID: 7820029

Claim:
A system of measuring pH of solutions, comprising: a first sensing unit provided in a solution, measuring pH of the solution to generate a first voltage; a second sensing unit provided in the solution, measuring pH of the solution to generate a second voltage; the first and second sensing units having the same time-drift effect; and a calibration device comprising: an offset voltage compensator outputting an adjustable compensation voltage; a first differential amplifier coupling the first voltage and the compensation voltage and outputting a third voltage; wherein the calibration device is configured to substantially zero the third voltage by adjusting the compensation voltage so that the third voltage is only responsive to the time-drift effect of the first sensing unit; a second differential amplifier coupling the second voltage and a reference voltage, outputting a fourth voltage; and a third differential amplifier coupling the third voltage and the fourth voltage to counteract the time-drift effect of the first and second sensing units, thereby outputting a fifth voltage corresponding to pH of the solution.