Checkline DFX-7 - Manual (Page 91)

DFX-7
Ultrasonic Flaw Detector
87
Example of Surface or Transducer Noise
Noise Blocked
The diagrams above illustrate a typical surface noise condition. Refer to the Noise
diagram: (A) refers to the noise in front of the actual back wall signal (C). Notice the
start position of the gate. As a result, the DFX-7 is detecting on the noise (A) as
shown at point (B). However, the true measurement should be taken at point (C).
Given the start, threshold, and gain levels, the amplitude from the noise is sufficient
enough to cause the DFX-7 to detect, or measure the noise rather than the true back
wall thickness (C). Therefore, the DFX-7 is making an incorrect reading at point (B).
Now refer to the Blocked diagram. The horizontal line at the top of (D), is GATE1.
The start of GATE1 has been moved just beyond the noise (A) to block the noise and
detect on the correct signal (C). Note: the DFX-7 will only detect on signals that are
located inside the dimensions of GATE1 (B). Therefore, the DFX-7 cannot see (A) at
all, with respect to the starting point of (B). Also notice, the position of the threshold
level with respect to the baseline (D), which represents the sensitivity setting of the
threshold. Extreme sensitivity is indicated by the bottom of the range (D) at the
baseline, and less sensitivity indicated by the top of the range at (D). Therefore, the
vertical height of GATE1 from the baseline, is the threshold level. The threshold level
can be increased to decrease sensitivity, or visa versa.
If the threshold level was increased in the Noise diagram, so that the vertical position
of GATE1 was higher than the amplitude of the noise (A), the DFX-7 would have
detected on the true back wall (C). Alternatively, if the gain level was decreased, the
signal amplitude of the noise (A) would have decreased below the threshold level,
and the DFX-7 would have also detected the true back wall (C). This example brings
all the fine adjustments into consideration, and demonstrates the versatility of having
a fully functional scope rather than a basic digital thickness gauge.
The procedures to adjust all of the features associated with the Gates are outlined
below: