DMTA-10004-01EN, Rev. D, November 2016
Chapter 13
236
13.8.2 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, resulting in a
hang-up condition (see Figure 13-14 on page 237). The interface blank should be set as
short as possible to avoid unnecessarily restricting minimum measurable thicknesses.
Table 22 Polarity of echoes
Measurement
mode
Echo 1 Echo 2
Mode 1
using contact
transducer
Back-wall echo 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
Interface echo normally
positive for high-impedance
materials such as metals and
ceramics, and negative for
low-impedance materials
such as most plastics.
Back-wall echo normally
negative unless the back-wall
echo represents the sort of
low-to-high impedance
boundary.
Mode 3
using delay line
or immersion
transducers
Interface echo normally
positive for high-impedance
materials.
Back-wall echo normally
negative except in some
special cases involving
difficult geometries where
phase distortion may cause
the positive side of the back-
wall echo to be better defined
than the negative side.
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