Reporting Defect Location in Guided Wave Testing

This post covers calibration, and it contains a piece of arithmetic that will change how you write a defect location.

A defect on a pipe can be located in four different ways:

• 82.3 feet from the MsS probe.
• 3 feet from the fifth pipe support.
• 9.5 feet from the third weld.
• 18.7 feet from the flange.

All four measurements are correct. Which one would you use to describe the defect location?

Now suppose the velocity carries a 2 percent error. That is normal when a nominal velocity is used rather than a measured one. A 2 percent velocity error gives a 2 percent distance error, so the further you measure, the more the error grows.

Measured from the probe, the defect lies somewhere between 80.65 and 83.95 feet. A range of 3.29 feet.

Measured from the nearest pipe support, it lies between 2.94 and 3.06 feet. A range of 0.12 feet, about one and a half inches.

Same defect. Same data. Same velocity error. The error is 27 times larger in one case than the other, and nothing changed except which feature you chose to measure from.

So the rule is simple. Find the geometric feature closest to the defect. Measure from it. And write the location in your report as a distance from that named feature — the third weld, the fifth support, the flange — not only as a distance from the probe.

Then the crew that goes out to find it can measure from something they can see.

The module also covers amplitude calibration using a weld, and is direct about its weakness. A weld is assigned a reflection coefficient of 10 to 25 percent. That range is the problem: a weld is not a manufactured reference, and its reflection depends on the cap profile and how much excess metal is there. It is the right method when nothing better is available, and the report should say it was used.