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Three-dimensional analysis of electrical capacitance tomography sensing fields

Electrical capacitance tomography (ECT) sensors can be categorized into three groups according to their axial guard electrode design, i.e. type 1, with earthed ring guards; type 2, with driven segmented guards; and type 3, without guards. In this paper, the sensing fields of the above three types of ECT sensor are analysed using a three-dimensional (3D) finite element method. Results show that for the same set of measurement electrodes, the 3D sensitivity distributions of different types of ECT sensor are very different - type 1 possesses the narrowest sensing space in the axial direction and type 2 the widest. These results are confirmed by the reconstructed images. The errors caused by approximating ECT sensing fields with 2D electric fields are also compared among these three types of ECT sensor.