Frequency of reversals is an important factor affecting the success of this method. With high frequency of current
reversals, the field generated in the part does not penetrate deeply into the part section since penetration decreases as
frequency increases. At a frequency of perhaps one reversal per second, penetration of even a large section is probably
near 100%. For moderate sized parts, the 50 or 60 hertz commercial frequencies of alternating current give quite
Limitations of Demagnetization.
Complete demagnetization is usually not possible, even though it is often specified. All practical demagnetization
methods leave some residual field in the part. Therefore, demagnetization is either the best effort that existing means
permit or reduction in magnetism to a residual level considered permissible in the particular part involved. It is
extremely difficult to bring the steel back to the original zero point by any magnetic manipulation. In fact, it is so
difficult that for all practical purposes, it may be said that the only way to completely demagnetize a piece of steel is to
heat it to its Curie temperature or above, and cool it with its length directed east and west in order to avoid
magnetization by the earths field. This method of demagnetization is never used because it is not only impractical but
such heating will alter the properties of the part.
It must be remembered that the earths magnetic field can determine the lower limit of practical demagnetization.
Long parts, or assemblies of long parts, such as welded tubular structures, are especially likely to remain magnetized, at
a level determined by the earths field, in spite of the most careful demagnetization technique.
Many articles and parts become quite strongly magnetized from the earths field alone. Handling of parts, such as
transporting from one location to another, may produce this effect. Long bars, demagnetized at the point of testing,
have been found magnetized at the point of use. It is not unusual to find that steel aircraft parts are magnetized after
having been in service for some time, even though they may never have been near any intentionally produced magnetic
field. Parts may also become magnetized by being near electric lines carrying heavy currents, or near some form of
The limits of demagnetization may be considered to be either the maximum extent to which the part can be
demagnetized by available procedures, or the level to which the terrestrial (earths) field will permit it to become
demagnetized. These limits may be further modified by the practical degree or limit of demagnetization that is actually
desired or necessary.
Selection of magnetic particle inspection equipment must consider the type of current to be used and the location and
nature of inspection. Magnetic particle inspection equipment must provide a rapid and convenient means for
magnetizing each part in a reliable and reproducible manner.
A variety of stationary, bench-type MPT units are available, with many characteristics that fit different testing
requirements. The smaller size units are used for small parts easily transported and handled on the unit by hand. The
larger ones are used for heavy parts such as long engine crankshafts, where handling must be by crane. Such units are
made to deliver AC or DC with various types of current control.
Mobile magnetic particle equipment delivering AC or half-wave DC magnetizing currents up to 6000 amperes are
available. Such equipment is used when it is necessary to conduct inspections in another shop area or in the field. This
type of equipment is sturdy and well suited for both fabrication and overhaul inspections.