Sondes (CF)

Choosing a Probe

Eddy Current (EC) testing differs from other types of non-destructive testing primarily in one key aspect: the equipment used, particularly the probe, which is specific to the nature of the inspection. This method is very flexible; however, to achieve good results, it's essential to carefully select the equipment according to the type of inspection. The information below helps guide the choice of a probe from this catalog; it details:

• The different types of probes available.
• How to define the type of inspection to be performed.
• The criteria for selecting a probe.
• How to make the right choice.

Types of Probes

Absolute and Differential

The winding of absolute probes detects the responses from the tested part; they are sensitive to both rapid and slow variations in the material.   Differential probes contain two or more windings, electrically connected in opposition. This configuration reduces simultaneous variations in both windings, such as those caused by temperature increases or changes in the geometry of the inspected part. Differential probes are only sensitive to rapid variations and are minimally affected by slow changes (including long flaws).

Reflection Probes

Reflection probes include a first set of windings that generate the initial magnetic field and a second set that senses the response from the material. The sensing coil can be absolute or differential. Reflection probes offer a wide frequency range and an excellent signal-to-noise ratio.

Shielded and Unshielded Probes

The main difference between shielded and unshielded probes is the extent of the magnetic field they generate. Unshielded probes create a wide field, which limits the number of passes required to inspect a part. They are typically used to detect larger cracks when the geometry of the part is uniform.   In shielded probes, the magnetic field is concentrated by placing a ferrite tube around the winding. This focuses the field into a narrow beam under the probe. As a result, shielded probes can be used near edges or when the part has abrupt geometry changes. They are also suitable when a ferromagnetic material is adjacent to non-magnetic areas. Don't hesitate to download our complete EC probe catalog (PDF format)

Frequency

The key decision when choosing a probe is identifying the optimal frequency.   This choice is based on:

• The nature of the material being tested
• The position of the flaw to be detected
• The type of generator used
• The required signal signature

  The frequencies listed in this manual are the central frequencies (F) of the probes. For probes used in bridge configuration, the spectral bandwidth is: 1/3 F to 3F.   The frequency determines the penetration depth of the eddy currents in the material. As frequency increases, penetration depth decreases. High-frequency probes are used for detecting surface flaws, while low-frequency probes are used for subsurface flaws.   There are three steps to determine the optimal frequency:

• Use the chart on the last page to select the appropriate frequency range.
• Ensure the chosen frequency is compatible with your generator.
• Occasionally, for non-ferrous materials, since the phase angle between the lift-off signal and the flaw signal is small, a slightly higher frequency than the calculated optimal can improve visualization.

Shielded/Unshielded

The difference between shielded and unshielded probes lies in the extent of the magnetic field they produce. Therefore, probe selection must take into account the geometry of the part and its surrounding environment.   Use a shielded probe to inspect areas:

• Near edges
• With varying thickness
• Where non-ferrous areas are adjacent to ferrous zones
• To detect short flaws

  Use unshielded probes when:

• The thickness is constant
• The flaws to be detected are long

  It’s also worth noting that unshielded probes require fewer inspection passes than shielded ones.

Probe Diameter and Configuration

  Probe choice is also dictated by the geometry of the part. Select the widest possible probe that still allows for effective flaw detection. Probes are available in a wide range of diameters, tip types, and angles.

Environment

  Probes are available for use in high or low temperatures, on greasy surfaces, or underwater. Probe tips can be protected for inspecting rough surfaces.

Cables

  A wide range of adapters is available to connect to virtually all types of generators. Cables are available in any length.

Special Probes

  If no standard probe meets your requirements, SOFRANEL can design a custom probe specifically for your application.