Design And Application Of A New C~4D Sensor Based On Impedance Elimination

Fluid widely exists in many industrial fields, such as petroleum engineering, chemical engineering, metallurgy engineering, energy engineering, food engineering, pharmaceutical engineering, etc. Conductivity is one of the important basic physical parameters of the fluid. Conductivity measurement has a great significance for industrial applications. However, the existing conductivity measurement techniques, which widely used in industrial processes, are contact detection techniques. The contact detection techniques have the disadvantages of polarization effect and electrochemical erosion reaction. These disadvantages limit the practical applications of the conventional conductivity measurement techniques.Capacitively coupled contactless conductivity detection (C4D) technique is a new kind of conductivity measurement technique, which can overcome the disadvantages of the contact detection techniques. However, the existence of the coupling capacitances has an unfavorable influence on the measurement performances (such as the measurement range, the resolution and the sensitivity) of a C4D sensor. To overcome the influence of the coupling capacitances, many methods have been proposed, including high frequency method, voltage difference method, resonance method and PSD (Phase Sensitive Demodulation) method. However, these methods still have some drawbacks. Some methods are difficult to implement. Some are limited in the selection of excitation frequency. So, to seek a new approach to overcome the influence of the coupling capacitances, more research should be undertaken.In this work, based on impedance elimination principle, a new C4D sensor is developed to overcome the influence of the coupling capacitances which exist in the conventional C4D sensor. And then, the new C4D sensor is applied to fluid conductivity measurement and parameter measurement of gas-liquid two-phase flow.The main research works and innovations of this dissertation are summarized as follows:(1) Based on impedance elimination principle, a new C4D measurement method is proposed to overcome the influence of the coupling capacitances. The new method eliminates the influence of the coupling capacitances by introducing external capacitive impedance and then differencing the impedance signal of the coupling capacitances with that of the capacitive impedance. Besides, the new method reduces requirements of the C4D sensor for excitation frequency in principle.(2) An impedance elimination module is designed to eliminate the influence of the coupling capacitances. The impedance elimination module consists of an impedance extraction unit, a capacitive impedance unit and a differential unit.(3) A new C4D sensor based on impedance elimination principle is designed. Its hardware system, which consists of an impedance elimination module, a sensing element, a signal processing circuit and a data acquisition module, is developed and the corresponding program is implemented.(4) Conductivity measurement experiments, frequency characteristic experiments and output characteristic comparison experiments are carried out in four pipes with the inner diameters of 1.8 mm,3.3 mm,5.4 mm and 7.6 mm, respectively. The experimental results show that the design of the new C4D sensor is successful. The influence of the coupling capacitances can be overcome and the conductivity measurement performance of the new C4D sensor is satisfactory. Compared with the reference values of a commercial contact conductivity meter, the maximum relative difference of the conductivity measurement is less than 5%. The new C4D sensor has similar output characteristics when the excitation frequency changes from 150 kHz to 400 kHz, which verified that the proposed method can reduce the dependence of the C4D sensor on the excitation frequency.(5) The new C4D sensor is applied to the parameter measurement of gas-liquid two-phase flow. A measurement system is established to implement the parameter measurements of gas-liquid two-phase flow. The preliminary experiments are carried out in two pipes with the inner diameters of 3.3 mm and 3.8 mm, respectively. The conductivity signals of different flow patterns are acquired and analyzed. The velocity of slug flow/bubble flow is measured. The experimental results show that the output signals of the new C4D sensor can reflect the flow characteristics of different flow patterns and the new C4D sensor can be applied to parameter measurement of gas-liquid two-phase flow.In summary, the new C4D method proposed in this work is feasible. The new method can overcome the influence of the coupling capacitances, improve the measurement resolution, expand the application range and reduce the dependence on the excitation frequency of C4D sensor. The design of the new C4D sensor based on impedance elimination is successful. The new C4D sensor has good measurement performance and can be used for fluid conductivity measurement and parameter measurement of gas-liquid two-phase flow.