Introduction to Eddy Current Technology

Authors Note:
As most of you are aware, I now work for Sonatest Plc, manufacturers of Ultrasonic NDT equuipment. At the time when I originally wrote this I was employed by Hocking NDT. Refer to their web-page for more details.

Other reputable manufacturers of Eddy Current instruments include Rohmann GmbHZetec  ( I currently work with Zetec products) and  Forster

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Historical Context

Basic Electrical Theory: Current Flow in DC circuits

Electromagnetic Induction

Current flow in AC Circuits - Impedance

Basic Eddy Current Testing, Pt 1

Simple Coil above a metal surface

Factors affecting eddy current response

Depth of Penetration

Basic Eddy Current Testing, Pt 2

The impedance plane

Coil Configurations:

Practical Testing

Typical Instrumentation

Meter/CRT Instruments

Operating frequency

Instrument set-up

Rate Filtering


Surface Crack Inspection

Non-Ferrous metal sorting

Sub-surface crack and corrosion detection

Heat Exchanger Tube testing

In-line inspection of steel tubing

Ferrous weld inspection


Written by Joseph M. Buckley,

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