TOFD is an advanced ultrasonic inspection technique that differs from conventional pulse echo technique. In TOFD diffracted signals from the edges of the defects are monitored & related to the true position and size of the defect as opposed to the reflection on defects according to a reference reflector.

The TOFD technique uses two probes in a transmitter - receiver arrangement. When sound is introduced into the mater salvia the transmitter the defect will oscillate. Each defect edge works as a source point of ultrasound signals. These very weak signals are called diffracted waves and their appearance does not relate to the orientation of flat or spherical defects. These diffracted signals are received via the receiver probe. The diffracted signals are evaluated to clear gray scale B-scan or 0-scan images.

The amplitude of the signal is not displayed, but the position of the signals on the time position of the signals on the time scale are displayed. Thus it is possible to determine the defect location exactly-length, & defect height.

Advantages of TOFD

• In contrast to the radiography method, planar defects & cracks, which are not perpendicular to the measured surface can be detected.
• Defect height can be exactly determined.
• The evacuation of areas because of radiation is not necessary.
• The inspect on results are immediately available, as is a permanent record & a permanent print as longitudinal or transversal projection of Thewlis available.
• Because of the high test speed the costs are less than those for radiography for wall thickness above 25 mm.
• The inspection can be performed above 200° C.
• TOFD saves costs, if applied during construction, since it is possible to distinguish pre-service and in - service defects. That means the unit can stay longer in production, and is safe.