DMTA-10043-01EN, Rev. C, July 2016
Chapter 10
62
so that the sound barrier material visible on the probe face is aligned
perpendicularly to the center axis of the pipe (see Figure 10-1 on page 62).
Figure 10
‑
1 Perpendicular to center axis alignment on pipe
It is possible that on some severely corroded or pitted materials there will be spots
where readings cannot be obtained. This can happen when the inside surface of
the material is so irregular that the sound energy is scattered rather than being
reflected back to the transducer. The lack of a reading may also indicate a
thickness outside the range of the transducer and instrument being used.
Generally, an inability to obtain a valid thickness reading at a particular point on a
test specimen could be a sign of a seriously degraded wall, which may warrant
investigation by other means.
•
Calibration
The accuracy of measurements is only as good as the accuracy and care with
which the gage has been calibrated. It is essential that the velocity and zero
calibrations be performed whenever the test material or transducer is changed.
Periodic checks with samples of known thicknesses are recommended to verify
that the gage is operating properly.
•
Taper or Eccentricity
If the contact surface and the back surface are tapered or eccentric with respect to
each other, the return echo becomes distorted and the accuracy of measurement is
diminished.
•
Acoustic Properties of the Material
There are several conditions found in engineering materials that can severely
limit the accuracy and thickness range that can be measured.
•
Sound Scattering
Sound scattering in some materials (notably certain types of cast stainless steel,
cast irons, and composites) occurs when sound energy is scattered from
individual crystallites in the casting or from dissimilar materials within the
composite. This effect reduces the ability to discriminate a valid return echo from
the back wall of the material and limits the ability to measure the material
ultrasonically.
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