Multi-frequency eddy current systems refer to equipment that can drive inspection coils at more
than two frequencies. This type of instrumentation is used extensively for tubing inspection in Power Generation and the Oil and Gas industries. Major advantages of this inspection are the ability to increase inspection information collected from one probe pull, comparison of same discontinuity signal at different frequencies, mixing of frequencies that helps to reduce or eliminate sources of noise and improves detection, interpretation and sizing capabilities.
A critical component of any eddy current examination is the ability to calibrate the unit based on reference standards manufactured from the same or very similar material as the test specimen. In the case of tubing inspection an ASME tubing pit standard is required.
The advantages of Eddy Current inspection are: sensitive to small cracks and defects, detects surface and near surface defects, immediate results are available, equipment is portable, minimum part preparation is required, probes do not need to contact the part and the ability to inspection complex shapes and sizes of conductive materials. The limitations of Eddy Current include; only conductive materials can be inspected, skill and training required is more extensive than other techniques, surface finish and roughness may affect the test.