DMTA-10009-01EN [U8778346], Rev. D, November 2016
Basic Operation
85
In some cases, an error can occur if you are using a D790 transducer to measure
thicknesses above 18 mm (0.7 in.). Typically, this error is due to a mode-converted
shear-wave echo that may appear before the second back-wall echo. If this unwanted
echo is larger than the second back-wall echo, the gage measures to it, which produces
a thinner reading.
You can usually distinguish the unwanted shear-wave echo from the correct back-wall
echo by examining the waveform display. The distance between the first and the
second back-wall echoes is the same as the distance between the zero thickness point
and the first back-wall echo. If there is an echo between the first two back-wall echoes,
it is probably a mode-converted shear-wave echo. Use the manual echo-to-echo
detection mode techniques and manually adjust the E1 blank to eliminate this error
(see “Blanking Adjustments in Manual Echo-to-Echo Detection Mode” on page 83).
Using the D797 transducer beyond 18 mm (0.7 in.) helps to eliminate the possibility of
this error.
In some cases, the second or third back-wall echo is smaller in amplitude than
subsequent echoes. This causes the instrument to give a double or triple reading. If
you are using a D790 transducer, this effect may occur around 5 mm (0.2 in.) on flat
smooth steel samples. If this occurs, it is clearly visible on the waveform display and
you can work around it using the manual echo-to-echo detection mode, or moving the
extended blank beyond the previously detected first echo.
When the 38DL PLUS cannot make an echo-to-echo reading, the
LOS
flag appears on
the screen. In this case, the waveform display shows that either no echoes are large
enough to be detected or that only one echo is detectable. In this last case, the echo-to-
echo detection bar begins at the detected echo but extends indefinitely to the right.
Increase the gain value to make a good echo-to-echo reading. If this does not help, you
can still obtain an approximate measurement by returning to the standard echo
detection mode.
5.6.3
Echo-to-Echo Mode Datalogger Flags
The following flags are used in the echo-to-echo modes in the first flag field of the
uploaded thickness table and in the comment note box at the top right corner of the
measurement screen:
•
E
: Automatic echo-to-echo detection mode
•
e
: Manual echo-to-echo detection mode
•
M
: Standard detection mode
•
l
: LOS in automatic echo-to-echo detection mode
Summary of Contents for 38DL PLUS
Page 6: ...DMTA 10009 01EN U8778346 Rev D November 2016 Table of Contents vi...
Page 8: ...DMTA 10009 01EN U8778346 Rev D November 2016 List of Abbreviations viii...
Page 24: ...DMTA 10009 01EN U8778346 Rev D November 2016 Important Information Please Read Before Use 16...
Page 26: ...DMTA 10009 01EN U8778346 Rev D November 2016 Introduction 18...
Page 46: ...DMTA 10009 01EN U8778346 Rev D November 2016 Chapter 2 38...
Page 56: ...DMTA 10009 01EN U8778346 Rev D November 2016 Chapter 3 48...
Page 102: ...DMTA 10009 01EN U8778346 Rev D November 2016 Appendix 94...
Page 106: ...DMTA 10009 01EN U8778346 Rev D November 2016 List of Tables 98...
