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4
SETTING-UP PROCEDURE
4.1
Test object surface preparation
In any ultrasonic measurement scenario, the shape and roughness of the test
surface are of paramount importance. Rough, uneven surfaces may limit the
penetration of ultrasound through the material, and result in unstable, and therefore
unreliable, thickness 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, sanders or
grinding machines may be used, after which the test surface should be cleaned of
resulting abrasive dust and chips.
Extremely rough surfaces, such as castings, will prove most difficult to measure
due to diffusion of ultrasonic waves.
Besides, rough surfaces contribute to excessive wear of the transducer,
particularly in situations where the transducer is "scrubbed" along the test surface.
Therefore, the transducers should be checked for wear on a regular basis.
4.2
Transducer selection
The thickness gauge is inherently capable of performing measurements on a
wide range of various materials. Different types of material, however, will require the
use of different transducers.
Choosing the correct transducer for a job is critical to being able to easily
perform accurate and reliable measurements.
The important properties of transducers should be considered when selecting a
transducer for a specific job.
Generally speaking, the best transducer for a job is one that sends sufficient
ultrasonic energy into the material being measured, so that a strong, stable echo-
signal is received by the thickness gauge.
Initial signal strength is largely a factor of the size of the ultrasound emitter in
the transducer. A large emitting area will send more energy into the material being
measured than a small emitting area.
Furthermore, as ultrasound travels through any material, it is partly absorbed or
scattered. The higher grain coarseness of the material, the more ultrasonic energy is
scattered.
The higher the frequency of ultrasonic waves, the more ultrasonic energy is
absorbed and scattered. While it may seem that using a lower frequency transducer
might be better in every instance, a higher frequency transducer would be the best
choice for testing objects with small thicknesses.
The physical constraints of measuring environment may also determine a
transducer's suitability for a given job. Some transducers may simply be too large to
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