T.O. 33B-1-14-40Figure 4-36. Single and Double Test Coil Configurations – Encircling Coils4.4.2.5 Configuration.Eddy current probes can also be classed according to the shape or some other prominent feature of the probe. Very thinprobes are called pencil probes. Probes with special electromagnetic shielding are called shielded or focused probes.Probes used in rivet or boltholes are called bolt hole probes. Certain types of probes with shaped ferrite cores may bereferred to as E-core, U-core, and pot or cup core probes.4.4.2.6 ApplicationsAndLimitations-SurfaceProbes.Most eddy current testing in the field is accomplished with surface probes. The surface probe is used on plate, sheet,irregularly shaped parts and in holes. The extent of the area to be tested by the probe is controlled by the coil diameterand by the presence of coil shielding. When the area to be scanned is large, pancake-type surface coils or overlappingmulti-coil probes can be used to reduce the time required to inspect the part. When small flaws must be detected, coilsas small as 1/32 inch in diameter can be used to examine limited areas.4.4.2.7 ApplicationsandLimitations-IDCoils.An inside diameter (ID) coil may be used on tubes, pipes, or other cylindrical parts where the geometry is regular andthe interior is accessible. The ID coil should nearly fill the part opening in order to provide a high fill factor formaximum test sensitivity. The use of ID coils can be restricted by bends or non-uniform diameters.4.4.2.8 ApplicationsAndLimitations-EncirclingCoils.Encircling coils are used primarily for inspecting rods, tubes, cylinders, or wire in manufacturing applications. Withboth encircling and inside coils the entire circumference of the specimen is evaluated at one time. Consequently, whilethe axial location of defects (along the part length) can be determined, circumferential location (around the part) cannotbe defined.
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