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Technical Papers
Fig. 5
we used a titanium tube 0.375 inch (9.5 mm) with a nominal wall measurement of 0.019 inch (0.048 mm). The UT instrumentation that we used was a Pico Pulser with a spike/square wave pulse rise time of less than 10 nanoseconds and a receiver bandwidth of at least 30 MHz. Figure 5. shows a photo of the Pico Pulser/Receiver. The line focused transducer that we used was broadband with a 20MHz center frequency. Since the wavelength of 20MHz is 0.012 inch, it is already less than 3/4 of a wall thickness of 0.019 inch (0.048 mm). Therefore using the Pico Pulser’s square wave pulser was more than an adequate choice for this wall measurement.
To obtain the measurement results, we rst made a test xture that insures the centering and stability as we discussed in Part 1 of this Series of 3 Articles in the “ITAtube Journal.” Figure 6. shows the xture was made, with a water chamber that mechanically centers three diam- eters of titanium tubing: 1/4”, 3/8”, and 1/2” (6.35 mm, 9.53 mm, & 12.7 mm). To x each tube diameter to the center of the chamber, a chord grip nut containing a centering bushing can be tightened or loosened to constrain the tube to the chamber center. The transducer is placed in the top cord grip nut where it can be adjusted to achieve the desired water path then xed by tightening the cord grip nut. The entire assembly, Figure 7., is then placed in a small immersion tank where water is circulated through the xture to surround the tube con- tinuously with water. The water provides the necessary water path that couples the sound beam to the tube wall being measured.
The electronic instrumentation used was the Pico Pulser/Receiver and an oscilloscope interconnected as shown in Figure 8. The Pico Pulser is a purchased compact pulser/ receiver preamp that enables you to send ultrasonic echo signals directly to a very sensitive oscilloscope with which you are able to view detailed echo information.
The oscilloscope trace in Figure 9. shows echo pairs, indicated by the red double arrow lines. There are 4pairs of echo multiples. The separa- tion of these echoes will enable the measurement to be made easily by ultrasonic wall thickness evaluation electronics. The time base at the bottom of the screen indicates that each major division of the graticule represents 100 nanoseconds. The distinct broad separations between the sharp negative going vertical pulses are approximately 150 nano- seconds. This represents a round trip time for 0.018 inch (0.46 mm).
Fig. 6
Fig. 7
ITAtube Journal No2/July 2018
44