In 1958, the Krautkramer company invented a series of standard curves using the principles of sound laws and the principles of sound intensity calculation, which is called DGS and is also known as AVG in German. In this diagram, each diagram represents a specific diameter of the Flat Bottom Hole (FBH) that the distances have gradually increased. The highest diagram is related to the return signals from the back wall of the calibration block.
The Y axis represents the peak amplitude and the X axis represents the Near Field coefficient of the probe. This means that it shows the height of the peak caused by the reference reflector in the distance of N.F * 1.2. Using these diagrams, it is possible to determine the signal caused by a discontinuity inside the piece, similar to the diameter of the FBH. Considering the diagrams Calculations of the divergence angle of a probe with a specific diameter and frequency, sound attenuation due to absorption and connection of the probe to the calibration block and the test piece are drawn. Different methods are used to draw the diagram for different probes.
By citing an example, we will see how to use the DGS/AVG diagram in ultrasonics:
A normal probe with a diameter of 10 mm and a frequency of 5 MHz was used to perform the test on a steel piece with a thickness of 100 mm. The size of a defect found at a depth of 42 mm is similar to which reference reflector?
First, we perform Near Field calculations N.F = 102/4*1.17 = 21 mm
Calculate the depth of the defect in terms of N.F and it is found to be at a distance of 2 N.F. (A=21/42=2) and mark 2 on the bottom horizontal axis.
According to the thickness, the distance from the back of the work piece is about 5 N.F and we draw 5 on the horizontal axis to the back wall diagram and read the number on the left side which is 10.
To calculate the reference dB, we set the signal received from the back of the work piece to a certain height of the screen, for example 60%, and write down the value of the reference dB (for example, 43 dB).
We bring the maximum signal received from the defect to the reference height (in this example, 60%) and write down the dB of the defect (for example, 45 dB).
The dB difference between the reference signal and the fault signal is equal to 2. (2=43-45)
By adding the sensitivity of the Back Wall signal obtained in paragraph 3, the real sound intensity number is obtained (V=2+10=12) and we mark 12 on the left vertical axis.
We connect the two specified points on the horizontal axis and the vertical axis, and now you can read the number written at the end of the diagram by following the meeting point of the two red lines on the diagram.
As shown in the diagram, our defect is equal to a FBH with a diameter of 0.4 mm.
