Optimal Design Of Capacitively Coupled Contactless Conductivity Detection System

Electrical conductivity is one of the basic physical characteristics of liquid, which can not only indicate the capacity to conduct electricity, but also reflect the concentration of the liquid composition. Therefore the detection of electrical conductivity plays an essential role in scientific research and industrial production.Capacitively coupled contactless conductivity detection (C4D) is a very important contactless detection method. The advantages of the technique over conventional galvanic contact conductivity detection include great flexibility, robustness and minimal maintenance demands. However, C4D is still in its infancy stage, which is still mainly used in the lab whose working environment is friendly. In addition, the majority of the sensor pipes of the exiting C4D systems are limited at the micrometer level. Based on the novel method, the paper aims to apply C4D to the routine industrial fields and the study is focused on the millimeter-level pipes. At first, the roles that the sensor parameters play on the detection are studied. Then the effects of grounded shield are analyzed. At last, a C4D system at the millimeter level is set up.The main works are listed as follows:(1) A C4D system was designed and set up. The system consists of five modules, which are excitation source, C4D sensor, AC current processing unit, data acquisition module and personal computer (PC). The data can be conveniently acquired, stored and displayed. In addition, a simulation program of C4D based on finite element method was completed using COMSOL Multiphysics and Matlab softwares.(2) The roles of the electrode length playing on the C4D detection performance were studied. The simulation and experiment of several millimeter pipes were performed. The results suggest that the increment of the electrode length can improve the detection performance. There exists an effective length in the system. When the length of electrodes is shorter than the effective length, the longer electrodes lead to larger sensitivity. When the length of electrodes is longer than the effective length, the sensitivity almost does not change with the increment of the electrode length.(3) The roles of electrode distance playing on the C4D detection performance were studied. The simulation and experiment of several millimeter pipes were performed. The results show that the output current can be increased and the detection sensitivity can be improved when the distance between two electrodes reduces. But the sensitivity curve becomes steeper and tends to zero sooner as the solution concentration increases, and the detection range of the system becomes narrower.(4) The grounded shield of C4D was investigated. The theoretical analysis and experiment results show that the shield can eliminate the influence of the capacitance coupled by two electrodes through air, which can improve the detection performance.(5) The sensor parameters were gained based on the above analysis and a C4D system was set up. The experiment results suggest that the detection performance of the millimeter-level system reach the level of the system reported in the literatures.