Low capacitance measurement is involved in many industrial applications, especially in the applications of electrical capacitance tomography (ECT). Most of the low capacitance measurement circuits employ an ac-based method or a charge/discharge method because of high sensitivity, high resolution, and immunity to stray capacitance; and its excitation or charge voltage are not more than 20 V. When ECT techniques for large industrial equipment such as blast furnaces or grain barns are explored, the existing methods for measuring low capacitance have some limitations. This paper proposes a high excitation voltage ac-based method for measuring low capacitance to improve the resolution of measurement. The method uses a high excitation voltage of several hundred volts and a transformer ratio arms as the C/V transducer. Experimental results indicate that the new method has a resolution of 0.005 fF, a good stability (about 0.003 fF over 4 h) and linearity (0.9992).

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The measuring concentration of Gas/Solid two phases flow is a difficult question in industrial process. The paper introduced an experimental research method and a system for measuring concentration of gas/solid two phases flow based on electrical capacitance tomography. The ECT system was 8 internal electrodes and an ac-based low capacitance measuring circuit. Two flow patterns, i.e. full and half field distribution, and 4 types of concentrations were used to be reconstructed by ECT system. Therefore, the experimental resultants showed that the relationship between the concentration and the average of grey level value of reconstruction image or, normal measured capacitance are monotonous and nonlinear, that means the method of measuring concentration of gas/solid two phases flow is feasible. In addition, the resultants also showed that the ECT system is ability to detect the blocking state of the pipe.

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