A strategy to screen capacitances of adverse effects on image reconstruction for ECT sensors mounted outside thick pipe wall

•The degradation of reconstructed images of ECT sensors outside thick walls is attributed to certain adverse capacitances.•These adverse capacitances are proved that only exist between electrodes with a circular central angle less than π/4.•A screening strategy is proposed, which enables reconstructing images of consistent qualities for varied wall thicknesses. Electrical capacitance tomography (ECT) is difficult to use non-invasively on pipes with thick walls due to the adverse influence of the wall on image reconstruction. This paper proposed a concise capacitance screening strategy for ECT sensors outside thick pipe wall, which enables reconstructed images of consistent qualities for varied wall thicknesses and applies to media with varied permittivity. Firstly, a systematic simulation study was conducted. The results show that the quality of reconstructions is dramatically degraded under particular wall thicknesses for a predetermined inner radius, and the thickness becomes smaller as the permittivity of the target medium increases. Through sensitivity analysis, the degradation is attributed to the measured capacitances between certain electrode pairs and meaningful images can still be reconstructed when these adverse capacitances are removed. These capacitances are proved to only exist between electrodes with a circular central angle less than π/4 by establishing an analytical model. Finally, the screening strategy is proposed, based on the analytical model and the simulation study, and performs well with simulation data. Experimental results demonstrate that this strategy can improve the quality of reconstructions under thick walls, where the relative image error can be reduced by up to 68 %.