What is Pulsed Eddy Current

Pulsed Eddy Current Testing (PECT) works on the principles of electromagnetic induction. In this technique, a coil inside the probe is excited with current pulse to generate a primary magnetic field near the probe. When a metal test specimen is close to the probe, electrical currents called ‘eddy currents’ are induced in the metal. These eddy currents generate a secondary magnetic field, which is sensed by receiver coils inside the probe. The received signal is displayed as a function of time to create an ‘A-scan’.

The PECT instrument is designed for the inspection of carbon steel and low-alloyed steels, which are Ferromagnetic. For magnetic test specimens, the eddy currents are concentrated on the surface directly after the magnetic pulse. Subsequently, the eddy currents diffuse into the test specimen, till the backwall is sensed. This results in a characteristic shape A-scan: a straight line, corresponding to the diffusion of the eddy currents, followed by curved section when the backwall is sensed. The transition of the straight section to the curved section is the ‘bending point’, which occurs at the ‘characteristic time’.

PECT can normally be applied to ferromagnetic steel with wall thickness (WT) between 3 mm (0.15”) and 102 mm (4”). A maximum surface temperature of up to 550°C (1020°F) may be inspected with the PECT technique, provided the PECT probe is kept below 80°C (175°F).

 

Why is Pulsed Eddy Current used

PEC Strengths:

  • Direct contact with the surface or surface preparation are unnecessary.
  • Measures the full wall thickness.
  • Sensitive to near-side and far-side defects.
  • Inspect through insulation, coatings, paints, concrete fireproofing, marine growth.
  • Inspect through aluminium, stainless steel, and galvanized steel weather jackets.
  • Inspect structures featuring chicken wire and rebars.
  • Unaffected by water or ice.
  • No costly or unnecessary insulation stripping.
  • No safety issues (unlike X-rays) and performed on working components.
  • Easy to use, fast learning curve.

 

PEC Weaknesses:

  • Affected by the edge effect (about the size of a probe’s footprint) near metallic structures.
  • Unable to discriminate between near-side and far-side defects.
  • Impossible to detect small pitting.
  • Under sizes flaws smaller than the probe’s averaging area (mitigated with the compensated wall thickness tool—see later in the training).
  • Difficult to use on elbows smaller than 200 mm (8”) in diameter.
  • Difficult to use on very fast materials (with characteristic decay rates less than 3ms).
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