Elcometer NDT MTG6 Guia Del Usuario página 39

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21 APPENDIX 1: PREPARING THE TEST SURFACE
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, a rotary sander 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.
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