Study On The Applications Of Capacitively Coupled Contactless Conductivity Detection Technique In Fluid Detection

Electrical conductivity is one of the basic physical characteristics of liquid, which can reflect the concentration of the liquid composition. Therefore the detection of electrical conductivity plays an essential role in researches on internal characteristics of multi-phase liquid. Compared with conventional electrode contact conductivity detection, capacitively coupled contactless conductivity detection (C4D) has the advantages of contactless detection, great flexibility and robustness. However, C4D is still in its infancy stage, which is mainly used in analytical chemistry filed. There are few reports about the applications of C4D in industrial process field and environmental monitoring field.The aim of this work is to study the applications of C4D in water quality measurement, mini-pipe and big-pipe flow measurement. The main works are listed as follows:(1) A novel C4D technology is developed based on phase sensitive demodulation, and the two-electrode C4D system is structured for mini-pipe flow measurement. Firstly, the single-phase fluid experiments is conducted with KCl solution, and test result demonstrates that compared with conventional C4D systems, this system can not only have high resolution and wide measurement range, but also obtain more information of the fluid. Secondly, the two-phase fluid experiments are conducted with gas-liquid, which obtains the conductivity signals of four typical flow patterns. Time-domain features and frequency-domain features are extracted from the obtained signals. Test result indicates that there are significant differences between the features about the signals of different flow patterns, which can reflect characteristics of gas-liquid flow availably.(2) The parallel resistance measurement principle of capacitively coupled electrical resistance tomography is proposed based on phase sensitive demodulation. Firstly, the measurement model of the novel sensor is analyzed. The new resistance-calculation method is studied. Then based on this model, the sensitivity fields are obtained using finite element method. Finally, the hardware design and the software programming are completed. After the debugging of the software and hardware of the system, the12-electrode prototyping system is achieved. Test result shows that the data acquisition speed can reach more than120frames per second, which means the proposed system improves the measurement speed of CCERT effectively.(3) A new non-contact measurement method is proposed for surface water conductivity measurement based on C4D to solve the problems which the sensor meets when is uesd for long-term and real-time on-line measurement in the application in severe outdoor environment. During the study, a new non-contact measurement sensor is developed and the system is manufactured adopting series resonance. In order to optimize parameters of the sensor structure and analyze the effect of impurities on the system, the model of the sensor is established using finite element method. Test results show that the measurement system is feasible and suitable for surface water conductivity measurement. The new non-contact measurement has the advantadges of sensor structure simplicity, non-contact electrodes and less interferences from impurities over other methods,,which can reduce maintenance workload of the sensor evidently.