對於尺寸相同但位於不同位置的缺陷，漏磁信號差異較大。為了對缺陷尺寸進行準確評估，首要任務是對管道內壁缺陷和外壁缺陷進行區分。常規漏磁檢測設備中，兩個磁鐵之間的距離相對較大，此時磁場能有比較強的穿透深度，對管道內壁和外壁進行均勻磁化。當磁鐵間距足夠小時，磁場會聚集在表面，增強外壁磁化效果，減弱內壁磁化效果。內壁磁化的減弱導致內傷信號的減小，因而使得內傷信號無法探測出。綜合使用兩個檢測單元，一個磁極間距較大，一個磁極間距較小，外傷在兩組檢測單元都有信號，而內傷只在磁極間距較大的檢測單元有信號，這樣就能對內外缺陷就行有效區分。

For the defects with same dimension but located at different places the MFL signal amplitude differs greatly. In order to get accurate evaluation of the defect dimension, the primary task is to discriminate the internal defects from external ones. For the conventional MFL apparatus, magnets distance is relatively large, so the magnetic field can penetrate to internal surface of the pipe and distributed uniformly in the pipe. When the two magnets are close enough, the magnetic field concentrates at the external surface, and is weakened at the internal surface. The reduction of magnetic flux density at internal surface causes the decrease of MFL signal, thus makes the internal defect unable to be detected. The combinational use of two inspection units, one with short magnets distance and the other with long magnets distance, can effectively discriminate the external and internal defects.