10
TO-BA-e-2020
The left figure shows a typical transducer from below. The two semicircles of the
contact surface are visible, as is the barrier separating them. One of the semicircles is
responsible for propagating ultrasound in the material to be measured and the other
semicircle is responsible for returning the echo to the transducer. When the transducer
is in contact with the material to be measured, the area just below the centre of the
contact surface is measured.
The right figure shows a top view of a typical transducer. Press the transducer upwards
with thumb or index finger to fix the transducer in place. Moderate pressure is sufficient
as the transducer only needs to be held immobile and the contact surface must beflat
against the contact surface of the material to be measured.
3.2 Condition and preparation of surfaces
In any ultrasonic measurement scenario, the shape and roughness of the
measurement surface are of utmost importance. Rough, uneven surfaces can restrict
the propagation of ultrasound through the material and lead to unstable and therefore
unreliable measurements. The surface to be measured should be clean and free of
small particles, rust or scale. The presence of such obstacles prevents the transducer
from contacting the contact surface correctly. Often a wire brush or scraper will help to
clean the surface. In extreme cases, rotary grinders or grinding wheels can be used.
However, care must be taken to avoid surface cracks that prevent proper coupling of
the transducer.
Extremely rough surfaces, such as the siliceous surface of some cast iron, are the
most difficult to measure. Such surfaces act on the sound beam like frosted glass on
light, the beam is diffused and dispersed in all directions.
Rough surfaces are not only an obstacle to measurement, but also contribute to
excessive wear of the transducer, especially in situations where the transducer
"scrubs" along the surface. Transducers should be checked regularly for signs of
uneven wear of the contact surface. If the contact surface is worn more on one side
than the other, the sound beam passing through the specimen may no longer be
perpendicular to the material surface. In this case, it is difficult to locate the smallest
irregularities in the material to be measured precisely, since the focus of the sound
beam is no longer directly below the transducer.
