Dakota Ultrasonics DFX-7 plus MANUAL1 - Manual (Page 76)

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CHAPTER TEN
DISTANCE, GAIN, SIZE (DGS)
10.1 Introduction to DGS (AVG)
The DGS (Distance, Gain, Size) method was introduced in 1958 as a way to
standardize the process of evaluating and sizing defects, also referred to by the
German acronym AVG. This method uses a combined empirical and mathematical
model to eliminate the need for a large range of reference standards, generally
required when using the DAC or TCG methods. DGS can be thought of as a
mathematically derived DAC curve, and used as an alternative replacement for DAC.
The concept of the DGS method is to compare the amplitude/gain of similar reflectors
at different distances and sizes, to mathematically approximate the size of the defect
using the effective probe diameter, frequency and near field length (NFL). The
reference to ‘similar reflectors’ assumes that the orientation of the reflector is the
same for all sizes, which is unrealistic to assume during actual field testing. The
diagram below illustrates the mathematically derived set of curves used for DGS: