Research On On-line Detector For PH/Conductivity

The pH and conductivity are the two commonly used parameters in solution domain,which widely used in industry,agriculture,food,medical,environmental protection,ocean detection and other fields.On many occasions,pH and conductivity need to be detected and controlled.The subject completed the research and design for a measuring instrument with three parameters,including pH,conductivity and temperature:(1)The system solutions design of the instrument.It includes power supply,singlechip microcomputer system,pH mesasurement unit,conductivity measurement unit,temperature measurement unit,4-20mA circuit,iolating circuit,etc.(2)The hardware circuit design of the instrument,A high input impedance op-amp CA3140 was employed in the pH measurement circuit,and a special processing was carried on the interface.The impedance measurement chip AD5933 by ADI company was embed to the conductivity measurement ciruit,and an expansion circuit was designed for it.It enabled the AD5933 to be applied to the impedance measurement of solution,and the two electrodes and four electrodes measeurment were achieved.DS18B20 was used in temperature measurement,and the main controller unit was microcontroller series of STM32F103 with ARM Cortex-M3 kernel.The voltage signal of pH was collected by the built-in AD of microcontroller,and the communication between AD5933 and microcontroller was completed by I2C interface.(3)The software design of the instrument.The software design of the instrument was compelted by C language base on the library functions.pH,conductivity,and temperature were dispalayed on host-computer software designed by Labview.(4)The measeurment experiment of the instrument.In the pH experiment section,electrode calibration and temperature compensation were compeleted.In the conductivity experiment section,experiments for two electrodes and four electrodes were carried on.For two electrodes experiment,excitation frequency selection experiment and calibration experiment were carried on.For four electrodes experiment,resistance was used in simulation experiment,and the experiment data proved the feasibility of the design of four electrode measurement circuit.The experiment indicated that the measurement error of pH was ±0.02,and the relative error of the conductivity with two electrodes was ±2%.