On-Line COD Monitoring Instrument Based On UV Absorptive Principle

With the increasing severe environmental pollution, it is significant todevelop COD monitoring instruments which are automated, real-time, simplyoperated and easily maintained. Although there have been COD measuringapparatus passed the national test, most of current machines based on chemicalmethod cannot meet the requirements, because of their long measuring-time, highfailure rate and secondary pollutions.In order to adapt to the complicated requirements in modern environmentdetecting industry and resolve the deficiency of current apparatus, after wereviewing the development of COD measuring techniques and existing CODmonitoring systems, the on-line monitoring instrument based on ultravioletabsorptive principle was developed.This system includes three sections: optical and mechanical system, electricand embedded system and software system. A low-pressure mercury lamp withthe wavelength of 254nm is used as the light source. The detectors are siliconphotoelectric diodes S2684-254 and S7686 made in Japan. By measuring theabsorbency of UV, the COD level of the testing water can be calculated. At thesame time, the absorbency of visible light is also measured and the system cancompensate the effect of turbidity so as to increase the stability and reliability.PC104 embedded system is chosen as hardware development plan for itssmall-size, high performance, good reliability and wide temperature range. A256M CF card is used as the data storage because the data can be kept for a longtime even power-off. HXL-511P with 12-bit ADC is used as data acquisition card;HXL-P582 is chosen in order to expand I/O channels; A 10.4 inch liquid crystaldisplay device and a wireless transmitting & receiving module are also includedin the system.The system takes Windows CE.NET as the operating system and EmbeddedVC++ 4.0 as software development environment, so it can satisfy the demands forgood stability and network.In this thesis, influences of background pollution and error sources are alsoresearched. Based on the research, the technology of eliminating background pollutions which utilizes distance adjusting is developed. In the software system,digital filter is also introduced to increase the certainty of measurement.With the help of the design mentioned above, the whole system has thefeatures of high response speed and accuracy, good stability, easy operation andnone secondary pollutions. It's a great improvement over the past apparatus andwill make full use of advantages in the future .Through long-time local test, theinstrument has reached the national standards.