Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Dead Zone - The interval following the surface of a test object to the nearest
inspectable depth. Any interval following a reflected signal where no direct
echoes from discontinuities cannot be detected, due to characteristics of the
equipment.
dead zone after echo and dead zone after initial pulse, both are common
phenomena. Actually the dead zone cannot be determined as a single figure
without additional parameters, hence the echo can be recognized, however,
signal quality is important. Useful parameters are linearity or signal in a nice
ratio that can describe the echo amplitude quality within a dead zone. For this
reason standards such as GE specifications are needed to check equipment
capability. The appearance of inference effects, within the dead zone, has to
be considered as well.
Definition by: http://www.ndt.net/ndtaz/content.php?id=103