Probes - Configurations
As mentioned on the previous page, eddy current probes are classified
by the configuration and mode of operation of the test coils.
The configuration of the probe generally refers to the way the
coil or coils are packaged to best "couple" to the test
area of interest. Some of the common classifications of probes
based on their configuration include surface probes, bolt hole
probes, inside diameter (ID) probes, and outside diameter (OD) probes.
Surface probes are usually designed to be handheld
and are intended to be used in contact with the test surface.
Surface probes generally consist of a coil of very fine wire encased
in a protective housing. The size of the coil and shape of the
housing are determined by the intended use of the probe. Most
of the coils are wound so that the axis of the coil is perpendicular
to the test surface. This coil configuration is sometimes referred
to as a pancake coil and is good for detecting surface discontinuities
that are oriented perpendicular to the test surface. Discontinuities,
such as delaminations, that are in a parallel plane to the test
surface will likely go undetected with this coil configuration.
Wide surface coils are used when scanning large
areas for relatively large defects. They sample a relatively large
area and allow for deeper penetration. Since they do sample a
large area, they are often used for conductivity tests to get
more of a bulk material measurement. However, their large sampling
area limits their ability to detect small discontinuities.
Pencil probes have a small surface coil that is
encased in a long slender housing to permit inspection in restricted
spaces. They are available with a straight shaft or with a bent
shaft, which facilitates easier handling and use in applications
such as the inspection of small diameter bores. Pencil probes
are prone to wobble due to their small base and sleeves are sometimes
used to provide a wider base.
Bolt Hole Probes
Bolt hole probes are a special type of surface probe
that is designed to be used with a bolt hole scanner. They have
a surface coil that is mounted inside a housing that matches the
diameter of the hole being inspected. The probe is inserted in
the hole and the scanner rotates the probe within the hole.
or Bobbin Probes
ID probes, which are also referred to as Bobbin probes or feed-through
probes, are inserted into hollow products, such as pipes, to
inspect from the inside out. The ID probes have a housing that
keep the probe centered in the product and the coil(s) orientation
somewhat constant relative to the test surface. The coils are
most commonly wound around the circumference of the probe so that
the probe inspects an area around the entire circumference of
the test object at one time.
or Encircling Coils
OD probes are often called encircling coils. They
are similar to ID probes except that the coil(s) encircle the
material to inspect from the outside in. OD probes are commonly
used to inspect solid products, such as bars.