T.O. 33B-1-1
3-58
Color contrasting is rarely used today, because the fluorescent materials now available solve the problem in a much
better way.
3.5.2.8.3
The ultimate in visibility and contrast is achieved by coating the magnetic particles with a fluorescent pigment (usually
available in wet method materials only). The search for indications is conducted in total or semi-darkness, using
ultraviolet light to activate the fluorescent dyes used. When indications glow in the dark it is almost impossible for an
inspector not to see them. Magnetically, these fluorescent materials are less sensitive than uncoated particles, but this
reduction in magnetic sensitivity is more than offset by the fact that patterns of particles can be readily seen even when
only a few such particles make up the indication. A fluorescent indication easily visible under black light is often quite
impossible to see when viewed in white light. The advantage in visibility and contrast of the fluorescent materials is so
great that they are being used in a very high percentage of all applications.
3.5.2.9
Dry Versus Wet.
The choice between the dry and wet methods is influenced principally by the following:
a. Type of Defect (surface or subsurface). For subsurface defects the dry powder is usually more sensitive.
b. Size of Surface Defect. The wet method is usually best for very fine and shallow defects.
c. Convenience. Dry powder with a portable half-wave unit, for instance, is easy to use on large parts in
the shop or for field inspection work.
The dry powder method is superior for locating defects lying wholly below the surface because of the high permeability
and the favorable elongated shape of the particles. These form strings in a leakage field and bridge the area over a
defect. AC with dry powder is excellent for surface cracks, which are not exceedingly fine, but it is of little value for
defects lying even slightly below the surface. When the requirement is to detect very fine surface cracks, the wet
method is considered superior regardless of the form of magnetizing current used. In some cases, direct current is
considered advantageous for use with the wet method to get better indications of discontinuities that lie just below the
surface. The wet method offers the advantage of easy complete coverage of the surface of parts of all sizes and shapes.
Dry powder is often used for spot inspections.
3.5.2.10
Visible Versus Fluorescent.
Selection of the color of particles to use is essentially a matter of obtaining the best possible contrast with the
background of the surface of the part being inspected. The differences in visibility among the black, gray, and red
particles are considerable on backgrounds which may be dark or bright and which may be viewed in various kinds of
light. Black stands out against most light colored surfaces, gray against dark colored ones. Red is more visible against
silvery and polished surfaces especially when the lighting is from incandescent lamps. If the indication is hard to see,
the inspector should try some other color of powder. In the case of the wet method, the ultimate in visibility and
contrast is obtained by the use of fluorescent particles. The fluorescent wet method has been used in constantly
increasing numbers of inspection applications for many years, principally because of the ease of seeing the faintest
indication.
3.5.3
Media Selection.
NDI laboratory generated results SHALL include supplemental information requesting that the following be included
on the purchase order or contract.
a. Suspension vehicle for magnetic particle inspection SHALL comply with DOD-F-87395. Table 3-5.)
