This differentiation can be used as a basis for

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This differentiation can be used as a basis for sorting. In Fig. 21, the shaded symbols represent the controls that are adjusted during each step of setup. As shown in Fig. 21a, a sample of steel A placed in the test coil causes a pattern to appear on the screen. The phase shifter and coarse sensitivity are adjusted until the entire pattern is displayed and centered. A fine sensitivity adjustment is used to bring the curve to a specific indication height called the absolute value. This adjustment lets all positions on the sensitivity selector be expressed as percentages of absolute value. If a second sample A 1 of steel A (Fig. 21b) is placed in the unoccupied difference coil and the sensitivity control is adjusted to a higher level, then a difference between the magnetic properties of samples A and A 1 will appear on the screen as indicated by the solid line in Fig. 21b. If A is then replaced by sample B 1 , a curve for this sample might be as indicated by the broken line in Fig. 21b. Next, sample A 1 is reinserted in the difference coil, replacing B 1 , and the compensator is adjusted so that curve A 1 becomes a horizontal, 340 Electromagnetic Testing F IGURE 21. Operating procedures and screen displays obtained when sorting two steels of composition A and composition B: (a) after phase and sensitivity adjustments; (b) second sample B 1 introduced; (c) sample A reintroduced; (d) curve B 1 centered; (e) curve A 1 symmetrically displaced; (f) amplification. Shaded symbols represent adjusted controls. Screen images show variation of permeability with magnetization (relative scale). (a) Sample A Compensator Amplifier Phase shifter (b) Sample A Sample B 1 Compensator Amplifier Phase shifter (c) Sample A Sample A 1 Sample A Sample B 1 Compensator Amplifier Phase shifter (d) Sample A Sample B 1 Compensator Amplifier Phase shifter (e) Sample A Sample B 1 Compensator Amplifier Phase shifter (f) Sample A Sample B 1 Compensator Amplifier Phase shifter Sample A 1 Sample A
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straight line (Fig. 21c). In the next step, curve B 1 is displayed by the phase shifter so that its peak or maximum lies in the center of the screen (Fig. 21d). In Fig. 21e, curve A 1 is symmetrically displaced downward with reference to curve B 1 by using the compensator. Finally the amplifier is turned up so that the area of the screen is fully used (Fig. 21f). Spread Bands When a large number of samples representing two steels of different chemical composition are sorted, minor variables in samples of the same steel result in a spread band or slight variations to either side of the principal curve. Slight differences in internal stresses or surface decarburization are among the factors that account for these variations. Typical spread bands made with identical instrument settings are shown in Fig. 22 for carbon steel and a sulfurized free machining steel. The bands shown in Fig. 22 reflect the spread obtained when 1000 samples of each steel were tested.
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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