GE General Electric Oxygen Equipment PT878 Benutzerhandbuch
Appendix D. Ultrasonic Thickness Gauge Theory of Operation
250
Transport® Model PT878 Portable Liquid Flowmeter User’s Manual
D.1 Factors Affecting Performance and Accuracy
A. Surface Condition - 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 sanded smooth in order to insure
proper sound coupling. It may also be necessary to remove paint if it has
been applied in thick coats, or if it is flaking off the metal. While it is 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.
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 sanded smooth in order to insure
proper sound coupling. It may also be necessary to remove paint if it has
been applied in thick coats, or if it is flaking off the metal. While it is 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 focussed 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.
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 focussed 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.
As with any difficult application, experimentation with actual product
samples is the best way to determine the limits of a particular
gauge/transducer combination on a given surface.
samples is the best way to determine the limits of a particular
gauge/transducer combination on a given surface.