A specialised NDT testing procedure needs to be followed to inspect these tubes, on-line as well as off-line.

The present paper describes the essence of the testing technique, especially, eddy current testing method for these tubes. Few double walled copper brazed tubes which were rejected during on- line testing were selected for the study. An off-line digital eddy current tester was used for testing purpose. The defect signals received were analysed in vector display mode using absolute and differential channels. The efforts were done to distinguish defect types based on the phase and amplitude of their corresponding test signals relative to phase sectors and alarm thresholds. The investigations were also carried out to find out interrelationship between the defect types and the mechanical properties.

The present study emphasizes the fact that the testing parameters could be different for different sizes of the tubes.