In large-scale factories or power plants metal tube clogging sometimes leads to serious accidents. However, metal tube testing for the detection of foreign objects requires a long testing time and considerable effort. Therefore, we proposed an original inspection method of a single metallic tube using electromagnetic wave reflection in the tube. However, for branched tubes, it is difficult to estimate the position of the clog by electromagnetic wave reflection. To evaluate the position of clog in the branched tubes, we proposed a new detection method using the group delay time in electromagnetic waves. The group delay time in electromagnetic waves is slightly affected by the bifurcation area. To verify the new method, the group delay time characteristics were measured in the branched tubes. The position of clog could be easily detected by the proposed method.
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