GE PT878GC User Manual download pdf (Page 285)

April 2004

C-2 Ultrasonic Thickness Gauge Theory of Operation

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.

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.

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GE PT878GC User Manual Page 285