T.O. 33B-1-1
3.6.9.1.1
When a field indicator is placed in a magnetic field, it responds to that portion of the magnetic field that
passes through the sensing element of the indicator. The indicator responds to the magnetizing force of the
leakage field passing through its sensing element, rather than the f lux density in the part from which the
leakage field emanates. When measuring the strength of the leakage field emanating from a part, the
indicator senses only the field at some distance from the part. This distance is from the center of the sensing
element to the bottom of the indicator when it is placed on the part’s surface. The f lux density of the field in
the part will be greater than indicated by the field indicator. How much greater will depend upon the
permeability of the part, shape of the part, and the effect of distance from the part to the sensing element in
the indicator. Since these variables have an effect on determining f lux density, it is recommended that the
field indicator be used only as a comparative indicator of the f lux leakage from a part. The sensing element
in newer indicators is of a ceramic-like material, which is very resistant to demagnetization. The indicator,
however, must still be kept away from fields that are strong enough to damage the needle because of rapid or
violent def lection beyond full-scale reading. Therefore, field indicators being used in support of magnetic
particle inspections SHALL be kept away from this area of inf luence. Field indicators, SHALL NOT be
stored within the inf luence of magnetizing or demagnetizing magnetic f lux.
3.6.9.1.2
Deleted
Change 3
3-85
