GE Phasor 16/16, Operating Manual

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5. Making Conventional Measurements
Page 78 PHASOR XS Operating Manual
5.14.4 Evaluating Results in DGS Mode
Once the DGS curve is recorded and displayed (by turning
DGS MODE on), echoes are automatically compared to
the recorded reference. There are three ways this com-
parison can be made and two additional DGS related
results that can be displayed:
A%rA —Amplitude of the signal crossing the A-Gate as a
percentage of the corresponding DGS curve amplitude.
dBrA —dB equivalent height difference between the signal
crossing A-Gate and the corresponding DGS curve height.
This value is shown in
Figure 5-10.
ERS —Evaluates the reflected echo and calculates the
Equivalent Reflector Size
Gt —DGS test gain, which initializes the DGS curve’s
max height at 80% FSH.
Gr —DGS reference gain, which represents the instru-
ment gain at which the reference echo’s peak reaches
80% FSH.
To aid in interpreting the DGS Mode’s display, the instru-
ment will display up to four curves representing fixed-gain
offsets from the DGS curve. These curves are enabled
and positioned (by specifying the dB-equivalent amount
they are offset above or below the DGS curve) by access-
ing the OFFSETS submenu in the DGS menu.
5.14.5 Locks, Error Messages
As long as a valid reference echo is stored, no functions
can be changed which could cause an incorrect DGS
evaluation. If an attempt is made to change such a func-
tion, the following error message will appear:
“Function Locked—DGS Reference has
been recorded”
The DGS evaluation must likewise be switched off and
the reference echo deleted in the case of selecting a new
probe, e.g. for a new test application.
5.14.6 Validity of the DGS Method
Echo amplitude evaluations according to the DGS method
are only reliable and reproducible in cases when:
• The reference echo is received from the test object
if possible. If this is not possible, it should be en-
sured that the reference block is made of the same
mateial as the test object.
• The evaluation is carried out using the same probe
which was also used for recording the reference
echo. Another probe of the same type can be used
after recording a new reference echo.
• Echo amplitudes for reflector distances smaller than
half of the probe’s near-field length are subject to
heavy variation – for physical reasons due to inter-
ference phenomena effecting the area. Thus evalu-
ation results may fluctuate more than the usually
permi2 dB. An evaluation according to the
DGS method is possible but not recommended for
this case.
5.15 dB REF Evaluation Mode
NOTE: Result-evaluating features, such as dB REF,
are first selected by the user via the EVAL MODE
function (located in the EVAL Menu). The selected
evaluation feature’s menu then appears in the HOME
Menu.
When dB REF is activated, the amplitude of the highest
echo in A-Gate becomes the reference echo against
which subsequent echo amplitudes are evaluated. At the
time of dB REF activation, the gain setting also becomes
a reference against which subsequent gain values are
compared.
Step 1: Access the SETUP Submenu (located in the dB
REF Menu).
Step 2: Locate the reference echo in A-Gate and ad-
just gain as required. Press
(two times) next to the
RECORD function to record the reference echo. The
function box will now indicate that a reference is stored
(
Figure 5-11 ). Also note the following:
• The instrument gain and the reference echo’s
amplitude now appear in the function box titled
REFERENCE.
• The instrument automatically switches the dB REF
MODE function to ON and the appears on the
display.
• Setting MODE to OFF allows you to disable the dB
REF feature without deleting the reference echo.
• Pressing next to DELETE REF, then confirming
by pressing , deletes the current reference.
It’s important to remember that the highest echo in A-Gate
and GAIN setting, when dB REF is selected, will become
the reference amplitude and gain value for as long as dB
REF is activated.
NOTE: The reference echo amplitude must not exceed
100% of full-screen height.