You place the probe, wait for the display to settle, and record the thickness. Lift the probe and put it back. The same number appears.
That repeatability is useful. It shows that you can obtain the same reading under those conditions. It does not, by itself, show that the reading represents the metal thickness you intended to measure.
Before that number goes into a report, the inspector needs to understand how it was obtained and what area it represents.
An ultrasonic thickness gauge calculates thickness from the travel time of sound. It needs an appropriate sound velocity for the material, along with the necessary zero adjustment for the measurement system.
A wrong velocity setting can produce a very repeatable wrong answer. The display may look normal because the instrument is performing the calculation it was given.
Consider a gauge left on a setup from an earlier job. Before collecting data, the inspector needs to establish that the material, probe, thickness range, and selected setup belong together. A familiar instrument is no reason to skip that review.
Checks on suitable reference material of known thickness help establish whether the system is performing as intended. Follow the inspection procedure and instrument instructions for those checks, including when a change in material, probe, or temperature requires another check.
Roughness, curvature, loose surface material, and coupling conditions can affect the sound entering the part and the return signal. When a reading settles after you move the probe, ask what changed. Did contact improve? Did you move onto a different local thickness? Are you still measuring the location the inspection requires?
A nearby spot that gives an easier reading may help explain the problem. It does not automatically replace the original measurement location.
A conventional measurement through a nonmetallic coating can include time spent traveling through that coating. The displayed value can therefore overstate the metal thickness.
Some instruments and techniques can separate or exclude the coating contribution. That capability has limits. Echo-to-echo measurements, for example, depend on obtaining suitable multiple backwall echoes, which may be unavailable in severe corrosion.
A through-coating setting still needs to be appropriate for the instrument, probe, material, and condition being examined. A stable display alone does not establish that the coating contribution has been excluded.
Unexplained readings call for investigation under the procedure. That may include waveform review by someone trained to interpret the signals and determine whether the measurement is valid.
A valid local thickness reading does not establish the minimum thickness of an entire pipe, tank, or structural member. Corrosion can vary across short distances. The measurement locations and coverage need to answer the inspection task.
There may also be areas where a valid reading cannot be obtained. That limitation needs to be addressed and reported. An unresolved location should remain visible in the inspection record, even when nearby readings are straightforward.
The report needs to preserve that context. Record the measurement locations, coverage, and limitations required by the procedure so the person reviewing the results understands what was examined.
In training, ask the student to explain the reading before accepting the number. What confirms that the setup suits the part? What area does the measurement represent? What would make the student stop and seek direction?
A steady reading is useful evidence. Knowing why it can be trusted, and where its limits begin, is part of doing the inspection. That builds on the difference between knowing an NDT method and knowing a particular application.
Explore our ultrasonic testing overview or TXNDT course catalog when planning training for the work you expect to perform.
