Dakota Ultrasonics DFX-7 plus MANUAL1 - Manual (Page 65)

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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%.