1. The principles of Eddy current testing for tube (ECT):

- Two coils are excited with an electrical current, producing a magnetic field around them.

Picture 1: Magnetic field generated from 2 coils.

- The magnetic fields penetrate the tube material and generate opposing alternating currents in the material. These currents are called eddy currents.

- Any defects that change the eddy current flow also change the impedance of the coils in the probe.

- These changes in the impedance of the coils are measured and used to detect defects in the tube.

Picture 2: Discontinuities change the eddy current flow.

2. Advantages and Disadvantages of UT:

- Advantages:

• Inspection speed up to approximately 0.5m/sec – 1m/sec.
• Distinguishes between ID and OD flaws.
• Reliability and accuracy of the test results.
• Can detect gradual wall thinning and localized flaws.
• Provides both phase and amplitude information.
• Permanent records can be obtained on test results.
• By using multi-frequency techniques, flaws under the support plates (baffles) can  be found and evaluated accurately. 

- Disadvantages:

• Limited to only non-magnetic tube material.
• Pin holes are difficult to detect and evaluate.
• Discontinuities adjacent to end sheets are difficult to detect.
• Reference standards needed for setup.
• Tubes must be cleaned.
• Requires high inspection skills for data analysis and evaluation.