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DMTA-10022-01EN, Rev. C, January 2015
Chapter 10 180
10.8.2 Interface Blank
The interface blank represents a blank or dead zone up to 20 µs long that follows the
leading edge of the interface echo. The interface blank is only available in mode 2 and
mode 3.
In mode 2, the interface blank prevents the detection of trailing lobes or cycles of the
interface echo which might otherwise be detected as back-wall echoes, which results
in a hang-up condition (see Figure 10-14 on page 181). The interface blank should be
Table 12 Polarity of echoes
Measurement
mode
Echo 1 Echo 2
Mode 1
using contact
transducer
The back-wall echo is
normally negative, except
when measuring material of
low acoustic impedance
bonded to a material of high
impedance (such as plastic or
rubber over metal), where the
echo is phase-reversed. Not applicable
Mode 2
using delay line
or immersion
transducers
The interface echo is
normally positive for high-
impedance materials such as
metals and ceramics, and
negative for low-impedance
materials such as most
plastics. The back-wall echo is
normally negative, unless the
back-wall echo represents the
sort of low-to-high
impedance boundary.
Mode 3
using delay line
or immersion
transducers
The interface echo is
normally positive for high-
impedance materials. The back-wall echo is
normally negative, except in
some special cases involving
difficult geometries in which
phase distortion may cause
the positive side of the back-
wall echo to be better defined
than the negative side.