location of the probe, probe handling, differences in tem- perature compensation, and standardisation. The thickness t is determined using Equation 1 by multi- plying the ToF data by the temperature-dependent material velocity v ( T ) and dividing by two:
An important note is that, like material velocity, these temperature corrections are approximate, as they do not account for variations in material properties due to the pro- duction process or changes caused by environmental con- ditions over the part’s service life. Corrosion impact on ultrasonic accuracy Permanently installed solutions that rely solely on tradi- tional methods for material velocity and temperature cor- rection do not inherently enhance the accuracy of thickness measurements. Furthermore, another source of variation arises from the nature of ultrasonic waves and their inter- action with irregular surfaces. Corrosion and erosion not only lead to metal loss but also cause the affected surfaces to become more irregular and rougher. As a result, ultra- sonic echoes reflected from a corroding surface are usually smaller in amplitude and broader compared to echoes from clean surfaces. In extreme cases, these irregular surfaces can create double or triple peaks in an echo, as illustrated in Figure 2 . Echoes from a relatively smooth surface are usually nar- row with a relatively high amplitude (top panel). In contrast,
(1)
Industry standards and literature offer the necessary cor- rection factors for various materials across different tem- perature ranges. Both portable and permanently installed instruments exhibit high accuracy in terms of ToF meas- urements and signal processing, with up-sampling further enhancing precision. However, the material velocity is typ- ically estimated, with standard values provided in industry guidelines. Since material velocity is temperature-dependent, addi- tional corrections are needed to determine thickness at room temperature when measurements are taken at tem- peratures above or below room temperature. Industry standards and literature offer necessary correction factors for various materials across different temperature ranges.
75
50
25
0
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Go
1W 1M 2M 6M 1Y 2Y 5Y 10/04/2021
30/06/2023
0.9130
0.8874
0.8618
Measurement time
Minimum Thickness
TC Thickness Temperature mm in All Thickness mm in
2020
2021
2022
2023
2024
Flip op trend: A-Scan - 03/02/2023 05:30:00
Gate A Gate B
Gates
mV
FSH [%]
μs mm
in
HF Recti ed
100
75
50
25
0
Inches
Figure 2 Effect of actively corroding surfaces on ultrasonic waveforms
100
PTQ Q4 2024
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