Particle concentration in the baths must be closely controlled if maximum sensitivity is to be obtained. Sensitivity is
lowered if concentrations are too low due to the lack of sufficient magnetic particles to be readily discernible. If
concentrations are too high, fine indications may be masked by heavy background accumulations.
Contaminants, particularly in wet baths, can result in lowered sensitivity. Lubricating oils and greases for example,
cause a blue background fluorescence that reduces contrast, causing fluorescent particle indications to be less visible.
Sensitivity of dry powders depends upon the type of powder selected, how carefully it is applied, and its color. Most
powders are made for general use and have a wide mix of particle sizes, so as to favor the detection of both fine surface
and deep subsurface discontinuities. A powder color is usually selected which will provide the best color contrast with
the color of the surface upon which it is being used. Care is needed in applying the powder. A light tossing and air-
blowing action is needed to allow the particles to migrate to and be held by the leakage fields at discontinuities.
Excessive application of powder can cause indications to be lost in background accumulation.
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
However, when the problem is to find very fine surface cracks, there is no question as to the superiority of the wet
method, regardless of the form of magnetizing current used. In some cases, direct current is selected for use with the
wet method to obtain the advantage of improved indications of discontinuities that lie just below the surface, especially
on bearing surfaces and aircraft parts. 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 very local inspections.
Circular magnetization is used for the detection of radial discontinuities around edges of holes or openings in parts. It
is also used for the detection of longitudinal discontinuities, which lie in the same direction as the current flow, either
in a part or in a part that a central conductor passes through.
A circular magnetic field is generated in a part whenever an electric current is passed through it or through an
electrical conductor that passes through the part. The circular field around the inside of the part will be wholly
contained within the part in the case of a concentric cylinder. No magnetic poles will be produced on the part. Poles
will be produced if the part is not a concentric cylinder, is irregularly shaped, or the path of the current flow is not
located on the part's geometric axis. The magnetic poles in these cases are caused by a relatively small portion of the
magnetic flux that passes out of the part and into the air that surrounds the part. The no pole condition in a concentric
cylinder occurs both while the magnetizing current is flowing and after current flow ceases. The part is thus residually
magnetized, but since no magnetic poles exist, the part appears to be in an unmagnetized state. However, if the part is
cut into, such as when a keyway is made, some of the field will pass out and over the cut, producing opposite magnetic
poles on each side of the cut. Such poles can hold chips or metal that can interfere with subsequent machining
operations or damage bearing surfaces. Care is needed in the case of circular magnetization, which may not be
detectable, and appropriate means to ensure demagnetization must be taken.
Two techniques can be used to produce circular magnetization in a part:
a. Direct Contact (Head Shot) Technique: Electric current is passed through the part itself.