DMTA-10022-01EN, Rev. C, January 2015Chapter 1018010.8.2 Interface BlankThe interface blank represents a blank or dead zone up to 20 μs long that follows theleading edge of the interface echo. The interface blank is only available in mode 2 andmode 3.In mode 2, the interface blank prevents the detection of trailing lobes or cycles of theinterface echo which might otherwise be detected as back-wall echoes, which resultsin a hang-up condition (see Figure 10-14 on page 181). The interface blank should beTable 12 Polarity of echoesMeasurementmode Echo 1 Echo 2Mode 1using contacttransducerThe back-wall echo isnormally negative, exceptwhen measuring material oflow acoustic impedancebonded to a material of highimpedance (such as plastic orrubber over metal), where theecho is phase-reversed.Not applicableMode 2using delay lineor immersiontransducersThe interface echo isnormally positive for high-impedance materials such asmetals and ceramics, andnegative for low-impedancematerials such as mostplastics.The back-wall echo isnormally negative, unless theback-wall echo represents thesort of low-to-highimpedance boundary.Mode 3using delay lineor immersiontransducersThe interface echo isnormally positive for high-impedance materials.The back-wall echo isnormally negative, except insome special cases involvingdifficult geometries in whichphase distortion may causethe positive side of the back-wall echo to be better definedthan the negative side.
