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

For a single crystal probe the length of the initial pulse is the dead zone and
any signal from a reflector at a shorter distance than this will be concealed
in the initial pulse. We deliberately delay the initial pulse beyond the left of
the time base, by mounting the transducers of a twin (or double) crystal
probe onto plastic wedges. This and the focusing of the crystals reduces the
dead zone considerably and it is only where the transmission and receptive
beams do not overlap that we cannot assess flaws.
A twin or double crystal probe is designed to minimise the problem of dead
zone. A twin crystal probe has two crystals mounted on Perspex shoes
angled inwards slightly to focus at a set distance in the test material. Were
the crystals not angled, the pulse would be reflected straight back into the
transmitting crystal.