Development Of The Monitoring System For Water Electrical Conductivity Based On Virtual Instruments

The electrical conductivity of solution is a basis physical-chemical character. The measure technic of electrical conductivity has applied widely in such fields as environmental inspection, medical sanitation, science research and products'quality check.Based on a large number of references and study of conductivity testing princilple, the author analyses the problems of conventional methods using AC excitation, and proposes a new method of electrical conductivity based on bi-directional pulsed voltage technic the width of pulsed voltage adjustable. Meanwhile, combined with Virtual Instrument technology which substitute software for some hardware circuit, a novel monitoring system for electrical conductivity is developed.The main items are listed as follows:(1) The design of the hareware of system is realized. The instrument adopts the bi-directional pulsed voltage as its exciting source, which can adjust width of pulsed voltage automatically according with different solution in order to eliminate both capacitive effect and weaken polarizational effect. The width of pulsed voltage and data acquisition are controlled by C8051F020 singlechip. Temperature measurement circuit, measuring range switch circuit, signal transform circuit, communication circuit are proposed, as well as others.(2) The design of the software of system is completed.C51 high-level language is used as the software of lower computer to write the modularization programme.The generation of bi-directional pulsed voltage sigcal, A/D convention, communication control is designed. Upper computer adopts labVIEW of Ni Instrument software as development platform in order to realize communication, data handling, result display and storage.(3) Calibration system of electrical conductance is completed and calibrate different measuring range is done by using curve fitting function of labVIEW. Software compensation is realized and precision of measurement is improved. A formula method ,which can implement temperature compensation automaticly, is adopted.The testing result shows that the system has such advantages as simple operation, powerful function, wide scope, high accuracy . It has a good using prospect.