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Electrical capacitance tomography (ECT) has been developed for two decades to visualize the internal behavior of industrial processes containing dielectric components. Three different ECT systems have been developed: (1) based on a charge/discharge capacitance measuring circuit, (2) based on an AC circuit and (3) based on an impedance analyzer. In this paper the operation principles, calibration methods and some performance of the three systems are compared. The experimental results prove that the AC-based ECT system is superior to the charge/discharge ECT system. Although the impedance analyzer based ECT system can provide the highest accuracy and the best image quality, it is bulky and expensive.