T.O. 33B-1-1
3-66
3.5.6.3.1
Black Powder Concentrate.
This is available as an oil- or water-suspendible dry powder. It is especially suited for finding fine cracks on polished
surfaces, such as bearings or crankshafts. It is the most sensitive of the non-fluorescent wet method powders for such
applications.
3.5.6.3.2
Red Powder Concentrate.
This is available as a reddish brown oil- or water-suspendible powder. The red color gives improved contrast and
visibility in situations where the contrast of the black powder is poor. This color tends to be more visible than the black
under incandescent light.
3.5.6.3.3
Suspension Characteristics.
Wet method particles may be suspended either in water or in a petroleum distillate. Water is initially cheaper, but
additions SHALL be made before it is a suitable medium for suspending the wet magnetic particles. Wetting agents,
anti-foaming materials, corrosion inhibitors, suspending and dispersing agents are all necessary and must be carefully
controlled. In order to assure proper control of the various conditioners, water as a suspending liquid SHALL NOT be
used unless adequate process control capabilities are present.
3.5.6.4
Particles.
Dry material concentrates to be used for water suspension must contain all of the extra ingredients necessary to make
the finished suspension. Cost of the concentrates is comparable for water or oil suspension.
3.5.6.4.1
The need to incorporate all of the special ingredients for water or oil suspension into the concentrate necessitates two
separate and distinct products. Water-suspendible concentrates cannot be used in oil. The various additives are
insoluble in oil and will not disperse the particles in an oil bath. The additions made to the concentrates intended for oil
suspension are not soluble in water. However, with suitable water conditioners, some of the oil-suspendible
concentrates can be used in water.
3.5.6.4.2
One outstanding characteristic of wet visible method particles is their extremely small size. These very fine particles do
not act as individuals but agglomerate into groups. Dry concentrates are almost always formulated to include all
required constituents.
3.5.6.5
Vehicle.
The bath liquid or vehicle may be either a petroleum distillate or water. Both require conditioners to maintain proper
dispersion of the particles and to permit the particles mobility to form indications on the surfaces of parts. These
conditioners are usually incorporated with the powders.
3.5.6.5.1
Petroleum distillates were the first choice as a suspension liquid. Significant characteristics for a suspension vehicle
are low viscosity, odorless, low sulfur content and a high flash point. The specifications for a suitable vehicle are given
in Table 3-5. Of these properties, viscosity is probably the most important from a functional standpoint. High viscosity
will retard the movement of particles under the influence of leakage fields, thus slowing the build-up of particles to
form indications.
3.5.6.5.2
Lighter distillates have even lower viscosities than those used, but they have other properties undesirable in a magnetic
particle bath. For example, lower initial boiling points accompany the lower viscosities and this results in faster
evaporation losses. In addition, a lower flash point also accompanies the lower viscosity with the resulting increase in
fire hazard. Inhalation of fumes from a light distillate can impair an inspector’s health. The odor of distillate can be a
distraction for the inspector and is associated with color and sulfur content.
