T.O. 33B-1-14-44produce a variable amplitude direct current signal. This is the most common type of detection on meter types of eddycurrent instruments.4.4.3.19 PhaseDetection.If an eddy current system is to be used to detect a variety of flaw conditions, phase angle measurements may be neededto determine the nature of the flaw condition. The information in the impedance diagram illustrates this fact. Decreaseof conductivity (i.e., cracks) and permeability changes could produce the same signal amplitude, and it would bedifficult to differentiate between cracks and normal permeability changes in a part. However, the phase angle of aconductivity change is very different from a permeability change if the correct test frequency is chosen. Using phasedetection techniques, it becomes a simple matter to detect the difference between permeability variations and cracks.This also applies to determining the depth of a flaw which is phase sensitive or separating lift-off effects from flawconditions. Phase sensitive detectors use a variety of techniques such as phase splitters, phase shifters, averaging, half-wave and full-wave detection, sampling, and subtractive and additive techniques. The presentation of the impedanceplane on waveform display eddy current instruments utilize two phase sensitive detectors to provide horizontal andvertical phase detection. This information is combined to produce a dot or point on the screen which represents therelative phase and amplitude of an eddy current signal. Some types of meter instruments utilize an adjustable phasecontrol or phase gate to allow only signal detection at a particular phase angle of interest.4.4.3.20 MultifrequencyTechnique.A single test frequency can provide phase and amplitude information for one material condition. If a variety ofconditions are to be detected, there must be a frequency for each condition. Multifrequency eddy current systems, forexample, can be used to detect the cracks in the presence of geometric changes in a complex part. Each condition to besuppressed must produce significant impedance changes for one frequency and less significant changes for the otherfrequencies used in the inspection. An example would be using a dual frequency inspection for subsurface corrosionwhile compensating for lift-off. A low frequency would be selected that would allow sufficient penetration to detect thecorrosion. Lift-off responses would also be present from this frequency. A higher frequency would then be used thatwould respond to lift-off and not have sufficient penetration to respond to the corrosion. In this simple example, lift-offcould be compensated for. The analysis of these signals can become extremely complex. Presently mostmultifrequency testing is limited to dual frequency testing and some three frequency testing. This technique is also usedin tube testing to discriminate between ID and OD defects in tubing.4.4.3.21 PulsedEddyCurrentTechniques.The pulsed eddy current technique is a non continuous wave test technique and also has multifrequency characteristics.The width of the pulse establishes the lower frequency limit while the sharpness of the pulse corners establish the upperfrequency limit.4.4.3.22 EddyCurrentInstrument.Table 4-5 lists current eddy current instruments and their applications, features and limitations. Figure 4-38 throughFigure 4-40 show examples of eddy current equipment.
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