This module answers one question. When you see a signal on the screen, what does its height mean?
The basic rule is one sentence. A guided wave reflects when it meets a change in the cross-sectional area of the pipe. A weld adds metal, so the area increases and the wave reflects. A defect removes metal, so the area decreases and the wave reflects. Coating and insulation change neither, so they do not reflect at all.
This module also explains where the percentage in your report comes from. The acoustic impedance of a pipe is the cross-sectional area times the density times the velocity.
At a defect, the density and the velocity do not change. Only the area can. So the reflected amplitude is proportional to the cross-sectional area that has been removed, and that proportionality is the entire basis for reporting a defect as a percentage.
The part of this module I also want to write about is a feature we removed from our own software.
A weld and a defect reflect with opposite phase. The weld adds metal, the defect removes it, so the two reflections come back inverted relative to one another. In principle, then, phase could tell you the reflector type automatically. Thus, we built that phase analysis section in the software. It worked. But later, we took it out. Why?
Part of the reason is technical. Phase can only be read reliably on a strong signal. On a weak one, close to the noise level, it is not dependable.
Now think about where you actually need the help.
You already know a weld or a flange from its position and its size.
Those are the strong signals. The ones you need help with are small signals, and those are exactly where phase stops working.
So phase-checking works where you do not need it and fails where you do.
The second reason matters more to me. Inspectors began relying on the phase label instead of looking at the signal itself. That is not analysis. It is accepting a verdict.
Removing the function sends them back to the tools that are reliable: the signal-to-noise ratio, the spectrogram, and how the signal behaves across different center frequencies. Those three build judgement that transfers to the next job. An automatic label does not.
One boundary is stated plainly throughout the module. Guided wave testing estimates defect size. It does not measure wall thickness. If you need a thickness, put an ultrasonic thickness meter on that spot. Guided wave testing tells you where to look; conventional UT tells you how deep it is. Neither replaces the other.