A large diameter range (15 mm to 600 mm) can be tested with this inspection concept. The ultrasonic probes are mounted in water-filled chambers underneath the tubes and the tubes pass them helically. The air-sprung test chambers follow the tube surface even in case of straightness deviations or ovality. The test chambers are raised for each tube and lowered again after the test.

With rotating examination, a wide helix is required for a high throughput. This is achieved by using wide probes (either several conventional probes in a common housing or phased array probes). The total sound field width corresponds to the helical feed of the tubes and therefore the possible throughput of the testing system is based on this. Normally separate chambers are provided, depending on the inspection task (longitudinal defects, transverse defects, oblique defects, laminations, wall thickness).