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An Innovative Tool For Magnetic Particle Bath Control
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Quality control of your NDT quality control procedures makes good sense. But the how-to-do-it is not always easy or available. Here is a clever idea that the author says can serve as a diagnostic tool for magnetic testing powders and baths. Now there is a plastic card for almost anything! Frank A. Iddings |
An innovative application of an often overlooked magnetic phenomenon can be used to simplify and to readily document the control over a magnetic particle inspection bath. The specialized magnetic encoding of the magnetic stripe on a unique card has been used by Circle Systems. Two companion magnetic stripe cards, Type A and Type B, can each instantly provide a "snapshot" of a magnetic particle bath. This snapshot allows inspection personnel to determine whether the bath meets operating requirements or it needs replacement or other corrective action.
Magnetic stripes are comprised of submicron sized particles of iron oxides or other specially formulated ferromagnetic materials. The stripe is embedded onto a plastic or vinyl card for encoding. The encoding process exploits the ferromagnetic properties of the tiny particles by arranging the direction of their magnetic domains (Figure 1). The encoder effectively reverses the direction of the domains of the particles as it proceeds across the length of the stripe. The unique patterns of these reversals allow credit cards, bank cards, and so on to be used to carry information to automated tellers and other card reading machines.
The beauty of using magnetic stripe cards lies in their simplicity.
Each reversal establishes an anomaly that effectively attracts the fine iron and iron oxide powders that are used as ingredients for magnetic particle inspection materials (Figure 2). When powders are applied over the stripe they make the encoded pattern visible, giving the stripe an appearance of a barcode (Figure 3). This general effect has been used in the recording industry to assist editors in pinpointing an area of tape which is of particular interest.
Figure 1 - The encoder is used to establish magnetic patterns in the magnetic stripe particles. Reversals in the magnetic field direction are represented by the arrows (Jorgensen, 1988).
The Type A and Type B magnetic stripe cards are made especially for the purpose of evaluating the properties of magnetic particle materials. Each card is encoded with the same pattern to make the best use of the space available on the stripe. The lines (i.e., reversals) are evenly spaced over most of the magnetic stripe, with a section that contains tightly spaced lines. After a powder or magnetic particle bath sample is applied over the stripe, an inspector can observe the consistency of the lines. Close observation will show whether the tightly spaced lines maintain their clarity.
The beauty of using magnetic stripecards lies in their simplicity. The cards require no onsite magnetization or demagnetization. They need only to be wiped off with a damp towel after use and may be stored virtually anywhere. Certification can be provided for each card. The cards are inexpensive and provide a high degree of both repeatability and consistency from card to card.
Figure 2 - In effect, the directions of the magnetic domains establish north and south poles in the stripe. Each polar reversal establishes a magnetic gradient. Ferromagnetic powders are attached to the gradiants.
Powders are simply applied over the stripe and the resulting lines are observed. Lines that are distinct and consistent show that the powder or bath is likely to be acceptable for use. Conversely, an unacceptable powder or bath would be demonstrated by the lines appearing broken, spotty, or otherwise inconsistent. If the lines in the tightly spaced section are not clear under close observation, there may be an over concentration of particles present. The cards allow permanent documentation, even for the purpose of making control standards. To do so, an imprint of the particles on the stripe can be made with transparent tape. The tape may then be transferred to a hard copy document.
The stripes on both styles of cards are coated to prevent any abrasion of the magnetic particle powders that may rub off the stripe particles. The high coercivity Type A magnetic stripe cards are well suited for general applications of dry method and larger sized wet method particle applications. The low coercivity Type B cards are more useful for high sensitivity and wet method materials. The main functional difference between the two styles of cards is that the lines on the Type A cards may attract particles that possibly would not be attracted to the Type B low coercivity cards. In effect the Type B cards are more discriminating in their evaluation of magnetic particle materials. Accidental erasure or alteration of the encoded pattern is unlikely with reasonable care.
Figure - 3 The encoded pattern of Type A and Type B magnetic stripe cards. The use of magnetic stripe cards does not yet appear in ASTM or other industry standards. Both ASTM E-709, "Standard Guide for Magnetic Particle Examination," and ASTM E-1444, "Standard Practice for Magnetic Particle Examination," address the use of test pieces for system quality control functions. It may be possible for a magnetic stripe card to serve as a test piece in certain situations. The card serves as a diagnostic tool in that it examines the magnetic particle bath or powder exclusive of the system's magnetizing apparatus. In practice it may be difficult for an inspection operation to locate and maintain suitable and consistent test pieces with known defects.
Reference
Jorgensen, Finn, Complete Handbook of Magnetic Recording, 3rd ed., 1988. Tab Books, Blue Ridge Summit, PA.
* Circle Systems, Inc., P.O. Box 1228, 479 W. Lincoln, Hinkley, IL 60520; (815) 286-3271; fax (815) 286-3352
Copyright © 1996 by the American Society for Nondestructive Testing, Inc. All rights reserved.
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