Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

As implemented in contemporary digital flaw detectors, DGS curves are
typically plotted based on a reference calibration off a known target such as a
backwall reflector or a flat bottom hole at a given depth. From that one
calibration point, an entire curve set can be drawn based on probe and
material characteristics. Rather than plotting the entire curve set, instruments
will typically display one curve based on a selected reflector size (registration
level) that can be adjusted by the user. In the example below, the upper curve
represents the DGS plot for a 2 mm disk reflector at depths from 10 mm to 50
mm. The lower curve is a reference that has been plotted 6 dB lower.
In the screen at left, the red gate marks the reflection from a 2 mm diameter
flat bottom hole at approximately 20 mm depth. Since this reflector equals the
selected registration level, the peak matches the curve at that depth. In the
screen at right, a different reflector at a depth of approximately 26 mm has
been gated. Based on its height and depth in relation to the curve the
instrument calculated an ERS of 1.5 mm.