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Also notable is the complete shielding above the

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Also notable is the complete shielding above the shield, as indicated by the abrupt decay to zero on the right side of the curves. Liftoff Characteristics The liftoff characteristics of shielded probes are shown in Fig. 27 and are compared to those of a simple absolute air core probe of identical dimensions in Fig. 28. In both cases, the frequency used is 100 kHz. The copper shield used here is a simple example of shielding. A similar shield could be used at a larger distance from the coil to reduce losses and reduce the decrease in probe field. As pointed out 147 Probes for Electromagnetic Testing F IGURE 25. Tangential component of the magnetic field for copper shielded probe at same locations as in Fig. 24. 1 2 3 4 5 6 7 –56.25 –37.5 –18.75 0 18.75 37.5 56.25 (–2.2) (–1.5) (–0.7) (0.7) (1.5) (2.2) Radial distance, mm (in.) Legend 1. 0.5 mm (0.02 in.). 2. 2 mm (0.08 in.). 3. 5 mm (0.20 in.). 4. 10 mm (0.40 in.). 5. 20 mm (0.80 in.). 6. 50 mm (2.0 in.). 7. 125 mm (5.0 in.). Normalized flux density B Z 1.0 0.75 0.5 0.25 0 –0.25 F IGURE 26. Normal component of magnetic field on vertical cross sections at six radial locations. 1.0 0.75 0.5 0.25 0 –0.25 Normalized flux density B Y 1 2 3 4 5 6 Legend 1. 8.5 mm (0.33 in.). 2. 9.5 mm (0.37 in.). 3. 11.0 mm (0.43 in.). 4. 15.0 mm (0.59 in.). 5. 50.0 mm (1.97 in.). 6. 120.0 mm (4.72 in.). –56.25 –37.5 –18.75 0 18.75 37.5 56.25 (–2.2) (–1.5) (–0.7) (0.7) (1.5) (2.2) Liftoff, mm (in.) F IGURE 27. Liftoff curves for shielded probes: (a) over nonmagnetic material; (b) over magnetic material. (a) 5.7 5.4 5.0 4.7 4.4 Impedance Z ) 0 3.0 6.0 9.0 12.0 (0.12) (0.24) (0.36) (0.48) Liftoff, mm (in.) 10.2 9.8 9.5 9.2 8.8 Impedance Z ) 0 3.0 6.0 9.0 12.0 (0.12) (0.24) (0.36) (0.48) Liftoff, mm (in.) (b)
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above, the field is greatly diminished by this type of shielding. In some applications, this is acceptable or even desirable (as in the case of testing nonconductive coating thickness). Otherwise, magnetic shielding should be used. Inside Diameter Probes In addition to surface probes, inside diameter or feed-through probes are widely used. There are two distinct types of inside diameter probes, absolute and differential. Absolute probes consisting of a single coil are very common but differential probes usually offer superior noise rejection characteristics and high sensitivity to sudden changes in geometry or material properties. Unlike surface probes, where the probe axis is frequently perpendicular to the test material, inside diameter probes usually lie parallel to the test object axis and therefore the dominant field distribution is called the lateral field . When inside diameter probes are used to test relatively thin walled tubes made of nonmagnetic materials, it can be expected that through-wall penetration will occur and both tube surfaces can be tested. With thicker walls or with magnetic materials, where the standard depth of penetration is significantly smaller than the wall thickness, the test becomes a test more of the surface than of the tube wall. Both absolute and differential inside diameter eddy current probes are treated extensively in the literature. 18-22 The treatment here will therefore be limited to basic examples of both as they relate to testing of tubes and circular cavities.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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