For this reason, the manufacturers of compressed gas cylinders, accumulators of large pressure vessels would like to see the hydraulic proof test replaced by a pneumatic test, with safety being ensured by suitable instrumentation. Acoustic emission is one technique that can be used for this purpose, as it has been demonstrated in recent developments:

• The method is emerging in the United States following more than seven years, research one more than 4000 pressure vessels. A procedure has been developed and regulations are in the process of being accepted. In France, the Air Liquide Company has tested a large number of compressed gas cylinders.

• The safety organisations are prepared to grant particular waivers: thus in Austria, hydraulic testing of accumulators is being replaced by a pneumatic test monitored by Acoustic Emission. Research showed that Acoustic Emission always detected any dangerous evolution.

• Along the same lines, CETIM and Air Liquide investigated the possibility of replacing the hydraulic proof test on very large pressure vessels in ferritic and austenitic steel by a pneumatic test monitored by Acoustic Emission.

This paper sets out the results obtained from tensile tests on test pieces (with and without effects) for determining the emissivity and criteria of the materials, and from a test carried out on vessels during a hydraulic test followed by a burst test.

This paper also presents the certification of operators in Acoustic emission in Europe and the State of standardization in this field.