Condition And Preparation Of Surfaces; Application Notes - Elcometer CG30 Operating Instructions Manual

16 CONDITION AND PREPARATION OF SURFACES

The shape and roughness of the test surface are of paramount importance when carrying out
ultrasonic thickness testing. Rough, uneven surfaces may limit the penetration of ultrasound
through the material, and result in unstable, and therefore unreliable, measurements.
The surface being measured should be clean, and free of any small particles, rust, or scale. The
presence of such obstructions will prevent the transducer from seating properly against the surface.
Often, a wire brush or scraper will be helpful in cleaning surfaces. In more extreme cases, rotary
sanders or grinding wheels may be used, though care must be taken to prevent surface gouging,
which will inhibit proper transducer coupling.
Extremely rough surfaces, such as the pebble-like finish of some cast iron, will prove most difficult
to measure. These kinds of surfaces act on the sound beam like frosted glass acts on light, the
beam becomes diffused and scattered in all directions.
In addition to posing obstacles to measurement, rough surfaces contribute to excessive wear of the
transducer, particularly in situations where the transducer is 'scrubbed' along the surface.

17 APPLICATION NOTES

17.1 MEASURING TUBING
When measuring a piece of pipe to determine the thickness of the pipe wall, orientation of the
transducers is important.
If the diameter of the pipe is larger than approximately 100 mm (4"), measurements should be made
with the transducer oriented so that the gap in the wearface is perpendicular (at right angles) to the
long axis of the pipe.
If the diameter of the pipe is small, two measurements should be performed, one with the wearface
gap perpendicular to the long axis of the pipe, another with the gap parallel to the long axis of the
pipe (Figure 7). The smaller of the two displayed values should then be taken as the thickness at
that point.
Perpendicular Parallel
Figure 7. Transducer positioning on small diameter pipe
17.2 MEASURING HOT SURFACES
The velocity of sound through a material depends upon the temperature of the material. As
materials heat up, the velocity of sound in the material decreases. In most applications with surface
temperatures less than approximately 100°C (~200°F), no special procedures are required. At
temperatures above 100°C (~200°F), the change in sound-velocity of the material being measured
starts to have a noticeable effect upon the accuracy of ultrasonic measurement.
At such elevated temperatures, it is recommended that the user perform a calibration procedure
(see "Calibration" on page 9) on a sample piece of known thickness, which is at, or near, the
temperature of the material to be measured. This will allow the gauge to correctly calculate the
velocity of sound through the hot material.
When performing measurements on hot surfaces, it may also be necessary to use a
high-temperature transducer. It is recommended that the transducer be left in contact with the
surface for as short a time as needed to acquire a stable measurement. While the transducer is in
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