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CHAPTER EIGHT
DISTANCE AMPLITUDE CORRECTION (DAC)
8.1 Introduction to DAC
A
DAC
curve is a gate with a variable threshold to compensate for a given reflector
type and size measured at different depths. Unlike a standard gate, which typically
has a fixed threshold level regardless of the depth, the
DAC
curve is more
representative of the reflector size at different depths. In order to generate a
DAC
curve, calibration standards must made with the same material being inspected, the
representative reflector size, and over a given thickness range. The
DAC
curve is
then used as an alarm to inform the user about the size of any given reflector at any
depth during the inspection process.
Once the
DAC
curve has been established, the user can display; -2dB, -6dB, -10dB,
-12dB, -6/-12dB, -6/-14dB, DAC or GATE at any time. The initial
DAC
curve
generated is always displayed in conjunction with any of the above
CURVE
options.
Additionally, the alarm can be set to
TRIGGER
on any of the curve options displayed.
Therefore, if the -6/-12dB curve option has been selected, the user can toggle the
TRIGGER
options: GATE, DAC, -6dB, or -12dB.
Note:
Based on the screen resolution of the
DFX-7
, the dynamic range of the
DAC
is approximately 18dB. As a result, inspection of any materials that attenuate more
than 18dB over the inspection range, the
DAC
feature should not be used, and
TCG
(Time Corrected Gain) should optionally be considered.
8.2 Additional Comments
Prior to proceeding onto the next section to create a
DAC
curve, a few additional
items will help with the users selection process in the menu items:
Up to 8 points can be used to generate a
DAC
curve.
3 or more points must be used in order to draw a
DAC
curve.
All the
DAC
curve or gate combinations only detect when the amplitude of the
signal is ‘at’ or ‘above’ the selected trigger option.
Any of the
DAC
curves or gate displayed, can be set as the trigger alarm
option.
The measure (meas) options are as follows:
dB –
%DAC – At the
DAC
curve represents 100%. Any signal generating an
amplitude below the
DAC
curve is less than 100%, and any above
greater than 100%.
