Dakota Ultrasonics DFX-8 plus MANUAL1 - Manual (Page 55)

DFX-8 Series Ultrasonic Flaw Detectors
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section 5.1 and 5.2, regarding the procedures to setup and calibrate the DFX-8. The
following are a list of additional items to consider when using an angle beam
transducer, as follows:
Calibration Standards:
There are a number of calibration block options available in a variety of common
material types, and according to specific code requirements. The user should
consider the application, codes applicable, and purchase the necessary calibration
standards.
Shear versus Longitudinal Velocity:
Unlike the previous straight beam calibration, which uses a longitudinal wave, an
angle beam transducer uses a shear wave. The following is a brief review of the
waves:
 Longitudinal –
o Particle motion is parallel to the direction of wave travel.
o Travels in solids, liquids, and gasses.
o Fastest mode of vibration.
 Shear –
o Particle motion is perpendicular to the direction of wave travel.
o Travels only in solids.
o Is approximately ½ the velocity of longitudinal waves.
o Has a shorter wavelength than a longitudinal at the same frequency.
Therefore, a two point calibration using an angle beam transducer will result in a
velocity approximately ½ the speed of the velocity calculated from the straight beam
calibration in the previous section.
Gain and Amplitude Ratio:
During an angle beam calibration the overall amplitude height of the waveform is
critical for “peaking up”. Therefore the maximum amplitude of the waveform must be
visible on the display in order to correctly position the transducer on the calibration
standard. The user should select a screen height and gain level that will enable the
entire waveform to be visible on the display.
Peaking Up the Signal Amplitude:
During an angle beam calibration, it’s important to peak up the overall amplitude of
the waveform to its maximum potential. The primary purpose is to be sure the
transducer is positioned properly on the calibration standard, that’s producing return
echoes at different distances. For example: If the operator is using a DSC block to
calibrate the velocity and zero, a DSC block will produce echoes at 1 & 5, or 3 & 7