Design And Implementation Of Low Concentration COD Detector Based-on LabVIEW

With the development of the industrialization process, the waste pollution problem caused by the industrial sewage draws more and more attention by the people. Water quality monitoring is an important foundation for water resources management and protection. Chemical Oxygen Demand (COD), as a commonly used water quality monitoring indicator, is used to monitor the organic content of the water.Therefore, the detection of COD in water is of great significance for the evaluation of the water pollution degree.This paper designs a low concentration COD detector, the hardware development environment of which is the virtual instrument of NI PXI series produced by National Instruments Corporation and the software development environment of which is Lab VIEW. The sensor used for detection is the COD sensor, which is developed independently by the school environment institute and this COD sensor is used to develop a new low concentration detector to meet the needs of national863project. The COD detector is based on the principle of the three-electrode electrochemical measurement and its potentiostatic voltage output is provided by the NI-4130and its response current is measured by NI-4071. The function of this COD detector contains COD detection, solution resistance measurement by the step voltage, curve comparison and cyclic voltammetry test. The COD detection can be set with two kinds of conditions which contain compensation and uncompensation, to detect COD content in the solution. To solve the problem of low concentration measurement which exists in the COD detector and reduce the influence of the test result owing to the existence of the solution resistance, the paper uses the the iR compensation method to improve the precision of voltage control, which premise is the known solution resistance. The step voltage test is used to measure the above solution resistance and we can calculate the solution resistance through integrating the current data which is collected by the COD sensor.The cyclic voltammetry test provides the cyclic voltammetry scanning function and can obtain the cyclic voltammetry scanning curve of the working electrode through collecting the measurement current emerged under the condition of constant speed triangle wave scanning voltage.The curve contrast function can implement the visual contrast of the multiple COD current data or multiple cyclic voltammetric curve in order to observe curve variation law.Due to the detector adopts the modularization structure design, both the hardware and software design have the strong flexibility and extensibility.The low concentration COD detector, which has been completely designed, can realize the measurement of the solution resistance and the test of the sensor volt-ampere characteristic curve. Based on the analysis of experimental data obtained through a variety of different concentration of glucose solution COD experiment, the result shows that COD sensor resolution is2mg/L. Based on above experimental data, This paper presents a low concentration piecewise line calibration method, which can reduce COD measurement error in the low concentration segment.