Section 2: Physics of Ultrasound

Dead Zone -The initial pulse is a technical necessity. It limits the detectability
of near-surface discontinuities. Reflectors in the dead zone, the nonresolvable
area immediately beneath the surface, cannot be detected (Figure
8-10). The dead zone is a function of the width of the initial pulse which is
influenced by the probe type, test instrument discontinuities and quality of the
interface.
The dead zone can be verified with an International Institute of Welding (IIW)
calibration block. With the time base calibrated to 50 mm, and the transducer
on position A (Figure 8-11), the extent of the dead zone can be inferred to be
either less than or greater than 5 mm. With the probe at position B, the dead
zone can be said to be either less than or greater than 10 mm.
This is done by ensuring that the peak from the Perspex insert appears
beyond the trailing edge of the initial pulse start. Excessive dead zones are
generally attributable to a probe with excessive ringing in the crystal.