Eddy Current Testing (ET) is

• the most common non-destructive method of testing thin-walled welded tubes
• a fast, efficient and accepted method of testing
• the base for welding factor 1.0 in Europe
• a substitute for hydrostatic testing
• is normally not used on tubes with wall thickness above 5 mm.

The method consists of primary coils in a system of coils that are energized with alternating current which creates a magnetic field in the tube. This magnetic field, in turn, gives rise to a voltage across two similar but oppositely wound secondary coils. When, for instance, a weld defect passes the system of coils, the voltage across the secondary coils will be different and consequently a defect signal is obtained. A defect signal of this nature from a natural defect can then be automatically compared with the defect signal from an artificial defect prescribed in the testing standards, i.e. a test defect. These test defects may be drilled holes of a certain diameter, or grooves of a certain length and width and with a depth which is usually expressed as a percentage of the wall thickness.

As a rule, Eddy Current testing takes place as final testing in a separate testing line. Here, testing of the entire tube cross-section takes place with surrounding coils, which may be either single or segment coils.

Illustration: Eddy current testing, arrangement sketch

Testing can also take place directly in the welding line. This testing can also be carried out with different types of surrounding coil systems, but testing with surface probes (which only cover the actual weld zone) is also employed.

Eddy Current testing is prescribed at two different levels with two different objectives:

• testing as an alternative to hydrostatic testing (e.g.
  EN 10246-2)
• testing of the weld in order to utilize the weld factor 1.0 (e.g. EN 10246-1).

As a substitute for hydrostatic testing, this method is approved by several authorities who insist only upon testing with surrounding coils. A surrounding coil test of the weld according to level 2, (as a basis for weld factor 1.0) is obviously also acceptable as a substitute for hydrostatic testing.

Certain authorities also accept testing in the welding line as a final test. This also applies to testing of the weld only, with surface probes. Testing in the welding line is also used as a purely internal manufacturing inspection method, enabling any welding defects to be not only detected but also eliminated as early as possible.