# Further Investigations into the Capacitive Imaging Technique Using a Multi-Electrode Sensor
## 5 Discussions and Conclusions  
In this paper, a new CI sensor with multiple electrodes was used to detect defects in the insulation layer and metal surface. The changes of the electric potential lines, which were acquired by FEM models, have validated the feasibility of multiple quasi-static electric fields detecting these defects. The MSDs of the multi-electrode sensor detecting the defects were acquired using the FEM models. The DCRs of the measured values obtained at the center of these two kinds of defects presented different variation patterns, which were verified by both FEM models and experiments. For the defect in the insulator layer, the DCRs of the measured values obtained at the center of the defect showed a monotonous decreasing trend with the electrode pairs changing from E21 to E31 to E41. While for the defect in the metal surface, the DCRs of the measured values obtained at the center of the defect showed a monotonous increasing trend with the electrode pairs changing from E21 to E31 to E41. These different variation patterns, which were explained by the trends of the changes of MSD, could be used to discriminate defects in the insulation layer and metal surface. In addition, analyzing all the detection results of the experiments can accurately compare the different depths of the defects in the insulator layer by using the multi-electrode sensor. For the surface defects, the surface depth was gradually decreasing from S1 to S2 to S3. Besides that, S1 and S2 were approximate with the sensing area of E31 and S3 was approximate with the sensing area of E21. For the buried defects, the buried depth was gradually decreasing from B1 to B2 to B3.  
It should be noted that, based on the principle of electrostatic shielding, the quasi-static electric field from the CI probe cannot penetrate into a conducting surface. Therefore, when applied to pipe inspection, the CI method is only sensitive for defect in the insulation layer and the outer surface of the pipe. In future, the change rules of MSDs and DCRs of detecting the defects in the insulation layer and metal surface using the multi-electrode sensor will be further studied when the lift-off changes and the influence of different regions of the negative MSDs on detection results will be further analyzed.  
Writing-Original Draft Preparation, Z.L.; Investigation, Z.L and K.W.; Writing-Review & Editing, X.Y.; Visualization, Y.G.; Supervision, G.C.; Project Administration, W.L.; Funding Acquisition, X.Y and W.L.  
This work was funded by the National Natural Science Foundation of China (No. 51675536 and No. 51574276), the Special national key research and development plan (No.2016 YFC0802303), the Major National Science and Technology Program (2016 ZX05028-001-05), the Fundamental Research Funds for the Central Universities (No.18 CX02084A), the Applied Fundamental Research Fund of Qingdao (16-5-1-12-jch), the Shandong Provincial Natural Science Foundation, China (ZR2016 EEQ26), the Postgraduate Innovation Project of China University of Petroleum (No.YCX2018056).  
The authors declare no conflict of interest.