Development Of Electrolyte Solution Conductivity Measurement System Based On Eddy Current Detection

Engineering conductivity measurement technology, with its wide scope, high accuracy, low cost, high security, convenience in continuous detecting, is more and more widely applied to soil salinity analysis, water environmental monitoring, industrial process control, multiphase parameter measurement, and chemical analysis, etc. Most of the objects to be measured in these fields exist in the form of electrolyte solution. In majority, engineering conductivity measurement is electrolyte solution conductivity measurement.So far, most of the electrolyte solution conductivity measurement techniques use contact electrode in the detection process. This method has problems like electrode polarization, corrosion, capacitance effects, etc. In recent years, some contactless measurement methods based on the principle of electromagnetic induction have been proposed. These methods can overcome the deficiencies of the contact measurement. However, there are still some problems: some sensors are intrusive since they need to alter the original pipe structure or to change the flow condition. The sensor structures are complex and they demand a high precision machining. The accuracy of the measuring system is lower than those which use the contact electrode.In this paper, the problem of conductivity measurement based on electromagnetic induction is studied. Sensor structure is optimized and design of the signal conditioning circuit is improved as well. An online electrolyte solution conductivity measurement system based on electromagnetic induction is developed. The main work is as follows:(1) The opposite-type eddy current sensor widely used in measurement of metal, alloy and other good conductors is applied in electrolyte solution that has a low conductivity. An online dual-coil pipe-attached sensor structure is proposed to simplify the tri-coil or quad-coil sensor used in electrolyte solution conductivity measurement. An online contactless and non-intrusive system for electrolyte solution conductivity measurement is developed.(2) Since the solution has low conductivity and weak eddy current, a high frequency sinusoidal excitation is used. The weak eddy current signal is picked up by a circuit based on a high-speed analog multiplier and it is very effectual. The equivalent circuit of the dual-coil pipe-attached sensor structure is analyzed as well as the interference channels. Using differential amplification to turn interference into common-mode signal in order to suppress it, a good anti-noise effect is achieved and the linearity of the measuring system is improved, thus offers the possibility for online measurement.(3) Initial experiments demonstrate that the sensitivity and precision of this measurement system is good even in a wide measurement range. This method is effective and achieves the expected design objective.