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

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.

ID 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.

OD 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.