During the construction of one of the nuclear reactors at Knolls Atomic Power Laboratory, a large amount of small diameter stainless steel seamless tubing was required. The extremely critical end use of the tubing imposed a mandatory quality level which could only be met by rigorous 100 per cent inspection of the tubing for both external and internal flaws. All tubing for this application was tested by hydrostatic pressure to a hoop stress up to 95 per cent of the yield stress, but there was reason to believe that a small percentage of tubes contained short, relatively deep defects which might initiate fatigue failure, and which were undetectable by the hydrostatic pressure test. The number of pieces involved was large, and it was of utmost importance that the method selected to test the tubes further be rapid, simple, free from ambiguous interpretation, and preferably one which would yield a permanent record for inclusion in the quality control exhibit required by our customer.