Capacitively Coupled Contactless Conductivity Detection System Based On Series Resonance

Capacitively coupled contactless conductivity detection (C4D) is a relatively new method. The advantages of C4D over conventional galvanic contact conductivity detection include great flexibility, robustness and minimal maintenance demands. However, C4D is mainly used in the lab research when the pipe dimension of the existing C4D system is micrometer. It hasn't been applied in general industries so far. The purpose of this paper is to find a new C4D method to improve the detection range, linearity and sensitivity, in order to be applied in traditional industrial field. The main works are listed as follows:(1) Based on the series resonance principle that when the system is on resonance, inductance would eliminate the effect of the capacitance then the system is resistive, an inductor was series between the signal generator and the excitation electrode to eliminate the effect of the capacitances coupled by electrodes and the liquid. Then the voltage on detection electrode may follow the change of the liquid conductance well. The advantages of the new method over conventional C4D method include larger detection range, better linearity and the sensitivity.(2) Designed two different shielding constructions to solve the effect of capacitance coupled by two electrodes through the air. The advantages of the two constructions were discussed through calculation and experiments. The double shielding construction may eliminate the effect of the coupled capacitance totally when the single shielding construction applied approximate calculation. But the construction of the double shielding would be more complicated and the requirements of the components would be higher. And when applied in some fields, there would not be many differences comparing the measurement results of the two constructions.(3) Designed a practical C4D system for industrial site. Many special process designs had been applied to satisfy the high requirements and adjusted the complicated environment of industrial site. Meanwhile, the problem of the floating voltages of the liquid in the ends of the pipe had been solved.