Eddy Current (ET)

What is eddy current?


Eddy Current (ET) is the nondestructive testing method used to inspect localized areas of metals for surface or subsurface flaws. This is done by using specialized equipment that generate magnetic fields and interact with conductive materials introduced onto the part being tested.


Non-destructive testing by Eddy Current relies on a primary electrical field generated by the testing equipment and a secondary electrical field known as Eddy Current passing thru the part being tested. This opposition creates an interaction that can be sensed and visualized onto the equipment operated by the technician. These interactions are shown in a CRT screen as a spike or squiggly line caused when reaching the edge of a crack, flaw, or even the edge of the part being tested.


One downside to performing Eddy Current is that flaws may not be visible to the naked eye, and therefore, it may be difficult for the customer to associate the squiggly mark on the CRT with the fact that a crack or flaw has been identified. This problem is known in the industry as ET Black Magic.

Eddy Current equipment comes in single or multi-frequency capabilities.


Single frequency, or conductivity and metal sorting testers with a digital readout, are considered “special use”, and they require less training to operate. These devices are used for basic surface and subsurface crack detection.


Multi-frequency equipment is used for testing heavier and/or larger parts such as pipes and tubes that require sensitivity on the surface and through the material.


High frequency equals less penetration and higher sensitivity. Low frequency equals more penetration and less sensitivity. This principle adds value to multi-frequency instrumentation that allows you to provide sensitivity and penetration at the same time. The selection of frequency is generally specified in the testing procedure being used to perform the inspection.


A common problem to Eddy Current Inspection comes when testing ferromagnetic materials because they generate erratic data. We can solve this problem by utilizing Remote Field Testing or (RFT).


RFT utilizes the properties of the magnetic material to its advantage. One known drawback is the loss of data when testing curved sections. We solve this problem by following the ET test with another method such as UTT (Ultrasonic Thickness Testing).


A common mistakes made by an untrained or wrongly trained technician is when he/she doesn’t have the appropriate calibration block and doesn’t perform post calibrations for every inspection.


The technician must post calibrate to ensure that the inspection is performed properly. This is why NDT technicians that use Eddy Current should be properly qualified and certified to a Level II before performing examinations. Technicians must pass all written and practice tests in accordance with codes such as SNT-TC-1A or NAS-410. These tests must be reviewed by a Level III technician. Facilities are supposed to have a primary Level III on file for code conformance.


If you do not have a Level III on file we can provide that service to keep you code compliant.


Here at Aqualified, we make sure to be transparent with our clients by providing (upon request) personnel records of the technicians who will perform the testing for you, allowing you to verify that they are following the applicable standard. This records should be provided to you within the hour of your request.


If you choose to hire a company other than Aqualified, please lookout for these common red flags found on some certification paperwork provided by less reputable NDT companies:


  • Lack of expiration dates
  • Not enough training or experience hours
  • Missing data on where they were trained and/or who trained them
  • Missing information on the methods or techniques the technician was trained on.

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