g. If the level or concentration of magnetic particles is above or below the range required,
correct by adding vehicle or magnetic particle powder respectively. Repeat step a.
through f. after making corrections.
h. Return contents of centrifuge tube to the unit suspension tank, and clean the tube
prior to next test. Dirt in the bath will also settle out and usually show as a separate
layer on top of the particles. The layer of dirt and lint is usually easily distinguisha-
ble, since it is of a different color and texture from the particles. The layer of dirt and
lint is usually easily distinguishable, since it is of a different color and texture from
the particles. Also easily distinguishable are iron peening shot and blasting grit; both
will settle faster and lie beneath the magnetic particles. The concentrates to be added
per gallon of bath, and the volume of solid materials, which settle out when the bath is
made up with these amounts of concentrates, should conform with the manufacturers
data supplied with the concentrate.
Preparation of Part Surface.
In general the same requirements apply for the wet method as for the dry technique. Dirt, corrosion, loose
scale, and oil or grease SHALL be removed. The oil bath will dissolve oil or grease but this builds up the
viscosity of the bath and shortens its useful life. With a water bath, oil on the surface of the part makes
wetting more difficult, although the conditioners in the bath are usually sufficient to take care of a slight
amount of oil. Excessive oil on part surfaces contaminates the water bath. Paint and plated coatings, if over
0.003 inch thick, may have to be stripped. Tests have shown that nonmagnetic coatings of any kind, in excess
of 0.003 inch in thickness, can seriously interfere with the formation of magnetic particle indications of small
Application of Suspension.
Many ways are used to apply magnetic particles in vehicle carriers. The methods range from a simple
pouring of a bath onto a part, to large industrial systems in which the bath is applied automatically, either by
immersion or f looding, then recirculated or reused. Occasionally small hand-held, lever-operated sprayers
Prepared bath is widely available in aerosol cans. Such cans, usually containing oil-based baths, are very
convenient to use for spot-checking, or small area tests in the field. They are often furnished in kits,
including a permanent magnet or electro-magnet yoke, which makes a portable package for small field
testing jobs or for maintenance testing around the shop. Various sizes of ordinary pressurized paint spray
tanks equipped with special guns are used, particularly with water-type baths. Aerosol containers SHALL be
checked for residual magnetism prior to being used to perform an inspection and, if necessary, demagnetized
to less than two increments on the magnetic field indicator. This is necessary to preclude the magnetic
particles from agglomerating and not being expelled from the container.
Shelf life dates on aerosol containers of magnetic particle materials is the
final date that the manufacturer will warranty its product. These products
SHALL be used after this date provided there is sufficient propellant remain-
ing in the container and they pass the system effectiveness check. Only
containers being used to perform inspections require testing.
One method practiced, mostly on small parts, is where the parts are magnetized one at a time, then placed in
a tray and immersed in a tank containing an agitated bath of magnetic particles. The parts must be placed in
the tray so they do not touch each other; or else non-relevant indications from magnetic writing may be
produced at the points of contact. Haphazard loading into a basket for immersion application SHALL NOT
be permitted. Both the concentration of the bath and the immersion time affect the production of indications.
If the leakage field at the discontinuity is weak, prolonged immersion permits more particles to come into the
inf luence of the field and makes the indication more visible.