Optimizing the frequency range of microwaves for high-resolution evaluation of wall thinning locations in a long-distance metal pipe

A vector network analyzer (VNA) and a self-designed coaxial-line sensor are utilized in this study to generate the time domain response of microwave signals. The frequency range of the signals is optimized by analyzing the time domain response. Thereafter, the pipe wall thinning (PWT) location is quantitatively evaluated by measuring signals at the optimum frequency range and extracting the time of flight (TOF) information corresponding to the PWT location. To approach a pipe with different PWT degrees and lengths, two brass pipes with inner diameters of 17.0 mm and lengths of 453 mm and 455 mm, and six brass joints having different inner diameters and lengths were used between the two pipes in turn to construct the combined pipes that were used in the experiment. By using the frequency range of 22.0–35.0 GHz, the maximum errors of evaluations are less than 0.38% and 1.04% of the full lengths of the pipes under test for the start and end points of the PWT sections, respectively. When the length of the PWT section increases, the evaluation precision also increases. This result indicates that a quantitative method for evaluating the PWT locations in the pipe under test with relatively high resolution and precision is established.