In many applications single absolute coils are used

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In many applications, single (absolute) coils are used and the coil impedance is monitored by using an alternating current bridge circuit. Homogeneity information is extracted from changes in the coil impedance. Double-coil probes consist of a driving coil and a smaller, concentric pickup coil. In this case, the induced voltage in the pickup coil is measured. The following discussion describes the application of the finite element technique to the analysis and design of both single-coil and double-coil surface probes. Double-coil probes are shown to be superior for the applications mentioned above because of better linearity of the liftoff curve and wider useful range. The rate of change in the induced voltage for a double-coil probe is shown to be larger than the rate of change in the impedance for single-coil probes with given parameter changes. Moreover, in the case of corrosion depth measurements, the noise generated by liftoff variations can be minimized with an appropriately designed double-coil probe. Some of the results presented have been verified experimentally. Single-Coil versus Double-Coil Probes The two probes considered here are shown in Fig. 3 as they relate to liftoff measurements. Figure 3a represents a small diameter absolute coil over a conducting surface. Because of the localized nature of the probe fields, the range of liftoff measurement is limited. A larger diameter coil such as the coil in Fig. 3b can extend this range but more sensitivity is obtained by introducing a small diameter pickup at the center of the larger coil (as in Fig. 3c). Because the flux lines through the pickup coil are essentially perpendicular to the conducting surface, the double-coil probe typically offers better linearity and a wider dynamic range. These effects can be seen in Fig. 4, where the effect of a flat bottom hole is shown. In Fig. 4a, there is little disturbance of the coil field from the localized field pattern. In Fig. 4b, the large diameter of the coil tends to mask the hole signal because there is little change caused by the hole. A double coil has the advantage of both of these characteristics 135 Probes for Electromagnetic Testing
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as shown in Fig. 4c. Because the bottom of the hole affects the flux passing through the pickup coil, it should be possible to measure a wide range of hole depths. Based on these considerations, three different absolute coils with 4, 10 and 30 mm (0.16, 0.4 and 1.2 in.) diameters plus a double coil consisting of an exciting coil 30 mm (1.2 in.) in diameter and a concentric pickup coil 4 mm (0.16 in.) in diameter were evaluated. In all cases, the coil thickness was 3.9 mm (0.15 in.). The performance of these probes was calculated for magnetic and nonmagnetic materials. The magnetic material was carbon steel with conductivity equal to 5 MS·m –1 and relative permeability of 50. The nonmagnetic material was a solid solution, strengthened, nickel chromium alloy (Unified Numbering System N06600) with a conductivity of 1.1 MS·m –1 . The magnetic material was tested at 1 kHz and the nonmagnetic material at 10 kHz. For each of the
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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