EDDY CURRENT TESTING
All kinds of Eddy Current Testing is based directly on Faraday’s law of Induction and Lenz’s law, which is actually a crucial extension of Faraday’s law. Faraday formed a turn of copper wire in a loop and showed that a voltage was generated in the loop when a bar magnet was inserted or removed from the coil. He also observed that when the magnet was stationary, no voltage was generated in the coil. He further observed that the voltage so generated was higher when the magnet was moved faster. These observations led him to the Faraday’s law of induction which states that: The electromotive force (EMF) induced in a coil of wire is proportional to the time rate of change of magnetic field coupling into that coil.
Eddy Current Sensors Two things are absolutely essential in any eddy current test instrument: A means of producing alternating magnetic flux (the Excitation) and a means of sensing the opposing flux that results from the flow of eddy currents (the Pickup). In practically all cases, the alternating magnetic flux is produced by a wound coil with one or many turns carrying current at test frequency. But there are several ways to sense the opposing flux. Most eddy current sensors take one of three forms or configurations – A probe suitable for scanning surfaces, an encircling coil for testing tubes or bars, or a bobbin coil suitable for testing tubes from inside. Single Absolute Coil The simplest sensor arrangement to take care of generation of flux and sensors.
Type of Sensors
Useful Tips:
- Thumb-rule: For meaningful eddy current inspection, the signal-to-noise ratio should be at least 3.
- Repeatability: 100 x Number of times Standard Defect Detected / Number of tube passes.