Home - Education Resources - NDT Course Material - EC Testing
 - Eddy Current Testing Introduction Basic Principles History of ET Present State of ET The Physics Properties of Electricity Current Flow & Ohm's Law Induction & Inductance Self Inductance Mutual Inductance Circuits & Phase Impedance Depth & Current Density Phase Lag Instrumentation Eddy Current Instruments Resonant Circuits Bridges Impedance Plane Display - Analog Meter Probes (Coils) Probes - Mode of Operation Probes - Configuration Probes - Shielding Coil Design Impedance Matching Procedures Issues Reference Standards Signal Filtering Applications Surface Breaking Cracks SBC using Sliding Probes Tube Inspection Conductivity Heat Treat Verification Thickness of Thin Mat'ls Thickness of Coatings Advanced Techniques Scanning Multi-Frequency Tech. Swept Frequency Tech. Pulsed ET Tech. Background Pulsed ET Remote Field Tech. Quizzes Formulae& Tables EC Standards & Methods EC Material Properties -

Pulsed Eddy Current Inspection

Conventional eddy current inspection techniques use sinusoidal alternating electrical current of a particular frequency to excite the probe. The pulsed eddy current technique uses a step function voltage to excite the probe. The advantage of using a step function voltage is that it contains a continuum of frequencies. As a result, the electromagnetic response to several different frequencies can be measured with just a single step. Since the depth of penetration is dependent on the frequency of excitation, information from a range of depths can be obtained all at once. If measurements are made in the time domain (that is by looking at signal strength as a function of time), indications produced by flaws or other features near the inspection coil will be seen first and more distant features will be seen later in time.

To improve the strength and ease interpretation of the signal, a reference signal is usually collected, to which all other signals are compared (just like nulling the probe in conventional eddy current inspection). Flaws, conductivity, and dimensional changes produce a change in the signal and a difference between the reference signal and the measurement signal that is displayed. The distance of the flaw and other features relative to the probe will cause the signal to shift in time. Therefore, time gating techniques (like in ultrasonic inspection) can be used to gain information about the depth of a feature of interest.

Additional Background Information on Pulsed Eddy Current Techniques