How to Select Eddy Current Probes

Eddy current probes induce detectable eddy currents in ferrous and nonferrous material for flaw detection, weld inspection, and other nondestructive test (NDT) applications.

Absolute Detection vs. Differential Detection

Two different detection formats are available: absolute and differential.

• Absolute detection uses a single pickup coil for flaw and crack detection and can detect both gradual and sharp changes.
• With differential detection, the detection of two pickup coils is compared for a greater resolution of sharp discontinuities or flaws. The differential nature makes this type of probe less suitable for detection of gradual changes in material.

Shielded Probes vs. Unshielded Probes

Eddy current probes can be either shielded or unshielded.

• Shielded probes provide greater focus of interrogating frequency by constricting the magnetic field. Typically, they are used for flaw detection around edges or for non-ferrous materials.
• Unshielded probes provide greater spread of magnetic field. They can thus cover a given area in fewer passes than an equivalent shielded probe, but should be used primarily where material geometry is constant and for detection of large cracks.

Reflection Basics

Eddy current probes can be reflection style devices. Reflection probes utilize a driver coil and a sensor coil. They are often differential, but may be absolute. The primary advantages of reflection probes are good accuracy (signal-to-noise ratio) and a wide band of operating frequency.  

Production Specifications and Features

One of the most important specifications for eddy current probes is the frequency of the coil or probe transmission. This value is typically specified as the center-band frequency.  Selection of frequency depends on many factors, such as material and subsurface depth of inspection interest.  In general, lower frequencies are used for greater subsurface depths and higher frequencies are used for detection of flaws or cracks near the surface.

Some common features for eddy current probes are adjustable size and and angled or curved tip that is often detachable or replaceable.