T.O. 33B-1-13-323.3.10.2.3Particle concentration in the baths must be closely controlled if maximum sensitivity is to be obtained. Sensitivity islowered if concentrations are too low due to the lack of sufficient magnetic particles to be readily discernible. Ifconcentrations are too high, fine indications may be masked by heavy background accumulations.3.3.10.2.4Contaminants, 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.3.3.10.2.5Sensitivity of dry powders depends upon the type of powder selected, how carefully it is applied, and its color. Mostpowders are made for general use and have a wide mix of particle sizes, so as to favor the detection of both fine surfaceand deep subsurface discontinuities. A powder color is usually selected which will provide the best color contrast withthe 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.3.3.10.2.6The dry powder method is superior for locating defects lying wholly below the surface because of the high permeabilityand the favorable elongated shape of the particles. These form strings in a leakage field and bridge the area over adefect.3.3.10.2.7However, when the problem is to find very fine surface cracks, there is no question as to the superiority of the wetmethod, regardless of the form of magnetizing current used. In some cases, direct current is selected for use with thewet method to obtain the advantage of improved indications of discontinuities that lie just below the surface, especiallyon bearing surfaces and aircraft parts. The wet method offers the advantage of easy complete coverage of the surface ofparts of all sizes and shapes. Dry powder is often used for very local inspections.3.3.11 CircularMagnetization.3.3.11.1 General.Circular magnetization is used for the detection of radial discontinuities around edges of holes or openings in parts. Itis also used for the detection of longitudinal discontinuities, which lie in the same direction as the current flow, eitherin a part or in a part that a central conductor passes through.3.3.11.1.2A circular magnetic field is generated in a part whenever an electric current is passed through it or through anelectrical conductor that passes through the part. The circular field around the inside of the part will be whollycontained within the part in the case of a concentric cylinder. No magnetic poles will be produced on the part. Poleswill be produced if the part is not a concentric cylinder, is irregularly shaped, or the path of the current flow is notlocated on the part's geometric axis. The magnetic poles in these cases are caused by a relatively small portion of themagnetic flux that passes out of the part and into the air that surrounds the part. The no pole condition in a concentriccylinder occurs both while the magnetizing current is flowing and after current flow ceases. The part is thus residuallymagnetized, but since no magnetic poles exist, the part appears to be in an unmagnetized state. However, if the part iscut into, such as when a keyway is made, some of the field will pass out and over the cut, producing opposite magneticpoles on each side of the cut. Such poles can hold chips or metal that can interfere with subsequent machiningoperations or damage bearing surfaces. Care is needed in the case of circular magnetization, which may not bedetectable, and appropriate means to ensure demagnetization must be taken.3.3.11.1.3Two 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.
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