April 2004
C-2 Ultrasonic Thickness Gau ge Theory of Operation
Factors Affecting Performance and
Accuracy
A. Surface Co ndition - Loose or flaking scale, rust, corrosion or
dirt on the outside surface of a test piece will interfere with the
coupling of sound energy from the transducer into the test
material. Thus, any loose debris of this sort should be cleaned
from the specimen with a wire brush or file before measurements
are attempted. Generally it is possible to make corrosion
measurements through thin layers of rust, as long as the rust is
smooth and well bonded to the metal below. Some very rough
cast or corroded surfaces may have to be filed or sand ed smooth
in order to insure proper sound co upling. It may also be necessary
to remove paint if it has been applied in thick coats, or if it is
flaking of f the metal. While it i s often possible to make corrosion
measurements through thin coats of paint (on the order of a few
thousandths of an inch or 0.1 - 0.2 mm), thick paint will attenuate
signals or possibly create false echoes, causing inaccurate
measurements.
Severe pitting on the outside surface of a pipe or tank can be a
problem. On some rough surfaces, the use of a gel or grease
rather than a liquid couplant will help transmit sound energy into
the test piece. In extreme cases it will be necessary to file or grind
the surface sufficiently flat to permit contact with the face of the
transducer. In applications where deep pitting occurs on the
outside of a pipe or tank it is usually necessary to measure
remaining metal thickness from the base of the pits to the inside
wall. There are sophisticated ultrasonic techniques utilizing
focused immersion transducers that can measure directly from the
base of the pit to the inside wall, but this is generally not practical
for field work. The conventional technique is to measure unpitted
metal thickness ultrasonically, measure pit depth mechanically,
and subtract the pit depth from the measured wall thickness.
Alternately, one can file or grind the surface down to the base of
the pits and measure normally.