Ultrasonic signals spread in metals without a considerable loss of energy. This physical effect is used for testing gas cylinders with ultrasonic. Ultrasonic transducers which are used as emitter and receiver send regular ultrasonic pulses into the surface of the gas cylinder. The ultrasonic waves spread and do not reflect if there are no flaws in the material. If there is a flaw in the material, parts of the sent signals are reflected. The reflected signals go back into the transducer and are displayed as a hit.
Four transducers are responsible for finding the longitudinal and transversal flaws. Two transducers search for transversal flaws (2+4) and two for longitudinal flaws (1+3). A fifth transducer (5) measures the wall thickness along the entire cylindrical length. Moreover, a sixth transducer may be added to test the wall thickness of the base.
The test head moves slowly from shoulder to base while the cylinder rotates on its own axis. As a result the complete cylindrical part and the transition area to the base and the shoulder are scanned.
The test head does not come into direct contact with the surface of the cylinder because the abrasion would be very high. Hence, ultrasonic waves are transferred onto the surface of the cylinder through water as a coupling medium.
Evaluation of Signals
Gates are set within the expectancy gate. If a flaw exceeds a gate, the indication has to be evaluated by an operator. The height of the gate corresponds to the artificial flaws given by the calibration.