Magnetic Flux Leakage (MFL) is a tube-testing technique designed primarily for the rapid testing of ferromagnetic tubes with non-ferromagnetic fins wrapped around them, such as those found in air fin coolers. Two powerful magnets produce a static magnetic field that completely influences the tube wall. When a defect (pitting, wall loss, etc.) is located between the two magnets, the magnetic flux in the tube wall is disrupted, and a small amount of flux leaks into the inner tube. These flux leaks are detected by coils placed between the magnets. The flux leakage variation induces current in the coils, thereby causing a signal output. This signal output can be used to provide information regarding any tube wall-thickness reduction. Magnetic flux leakage (MFL) is primarily used for the inspection of air fin coolers, but it can also be used to inspect bare tubes with diameters greater than one inch (2.5cm).