| In this paper , an intelligent acidity meter is designed which takes glass electrode as its sensor and measures acidity of solution through potentiometry. A resolution is put forward to eliminate the influence caused by three factors,i.e. solution temperature, high inner impedance of glass electrode and deviation of electrode.In order to compensate the temperature of acidity sensor, electronic bridge and differential amplifier are designed to detect and amplify the signal of platinum thermistor sensor. A resolution method in software is applied to calibrate zero point and full scale of the circuit.Software approach to nonlinear correction for sensor and circuit is presented. Therefore system error is eliminated ,and measurement accuracy of temperature is improved. The signal of acidity electrode is detected by a differential amplifier circuit, which consists of operational amplifiers TLC2254 with high input impedance and low drift. In order to improve the circuit's input impedance, which is affected by insulating resistance of electrode leading wire,terminals and printed circuit board, electrode is connected to the meter through insulating wire of high impedance which is directly welded in the printed circuit board with insulating ring .The deviation of electrode is corrected by software and the algorithm is given.The meter is designed with isolated RS485 series interface and standard communication protocal compatible with ADAM4000 and can be connected to control network directly.Results of Experiments show that accuracy of temperature and acidity arrive at 0.2 and 0.02pH respectively. |
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