Figure 4-36. Single and Double Test Coil Configurations Encircling Coils
Eddy current probes can also be classed according to the shape or some other prominent feature of the probe. Very thin
probes 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 be
referred to as E-core, U-core, and pot or cup core probes.
Applications And Limitations - Surface Probes.
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 diameter
and by the presence of coil shielding. When the area to be scanned is large, pancake-type surface coils or overlapping
multi-coil probes can be used to reduce the time required to inspect the part. When small flaws must be detected, coils
as small as 1/32 inch in diameter can be used to examine limited areas.
Applications and Limitations - ID Coils.
An inside diameter (ID) coil may be used on tubes, pipes, or other cylindrical parts where the geometry is regular and
the interior is accessible. The ID coil should nearly fill the part opening in order to provide a high fill factor for
maximum test sensitivity. The use of ID coils can be restricted by bends or non-uniform diameters.
Applications And Limitations - Encircling Coils.
Encircling coils are used primarily for inspecting rods, tubes, cylinders, or wire in manufacturing applications. With
both encircling and inside coils the entire circumference of the specimen is evaluated at one time. Consequently, while
the axial location of defects (along the part length) can be determined, circumferential location (around the part) cannot