T.O. 33B-1-13-119Table 3-9. Magnetic Field Strength and Duration Recommendations.INSPECTION AREAFIELD STRENGTH(GAUSS)MAGNETIZATION DURATION, EACHDIRECTION (SECONDS)50 to 75 15Hole (bare) 25 to 50 3015 to 25 12015- to 200 30100 to 150 90Surface (bare) 50 to 100 300 (5 minutes)40 to 50 450 ( 7 1/2 minutes)20 to 40 900 (15 minutes)f. Determine the direction of the field. Since cracks and other flaws are displayed more strongly whenthey lie perpendicular to the magnetic lines of force, the magnetism should be applied from twodirections to increase reliability. Usually this is accomplished by magnetizing in one direction and thenrotating the magnetization source 90 degrees and magnetizing again. When the direction of a suspecteddefect is known, such as in thread roots, only one magnetizing direction is required.g. Measure the magnetic field strength using a dial probe or a gaussmeter as follows:(1) The dial-probe is used by placing the probe into the test hole or on the test surface and slowlyrotating the probe counterclockwise. The maximum dial reading is the magnetic field strength.(2) The gaussmeter has interchangeable probes to permit measurement of the magnetic field eitherparallel or perpendicular to the axis of the probe. The probe is placed in the hole or on thesurface as shown in Figure 3-77.
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