T.O. 33B-1-1
3-119
Table 3-9. Magnetic Field Strength and Duration Recommendations.
INSPECTION AREA
FIELD STRENGTH
(GAUSS)
MAGNETIZATION DURATION, EACH
DIRECTION (SECONDS)
50 to 75
15
Hole (bare)
25 to 50
30
15 to 25
120
15- to 200
30
100 to 150
90
Surface (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 when
they lie perpendicular to the magnetic lines of force, the magnetism should be applied from two
directions to increase reliability. Usually this is accomplished by magnetizing in one direction and then
rotating the magnetization source 90 degrees and magnetizing again. When the direction of a suspected
defect 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 slowly
rotating 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 either
parallel or perpendicular to the axis of the probe. The probe is placed in the hole or on the
surface as shown in Figure 3-77.
