26
en
For full details of the Elcometer NDT range of transducers contact your local Elcometer NDT
supplier, or visit the Elcometer NDT website www.elcometerndt.com
19 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.
Thin materials
PULSE-ECHO
(P-E)
COATING OFF
High frequency trans-
ducers are required;
typically the 7.5MHz
and 10MHz models with
extra resolution.
The higher frequencies provide
greater resolution and a lower
minimum thickness rating overall.
High tempera-
ture
PULSE-ECHO
and
ECHO-ECHO
Special 2.25MHz and 5
MHz High temperature
transducers are
required.
Echo-echo mode will eliminate
error caused by temperature vari-
ations in the delay line of the
transducer.
Noisy material
Select a higher fre-
quency transducer to
reduce this noise -
7.5MHz and higher for
better resolution.
Materials such as titanium, stain-
less steel, and aluminium may
produce surface noise. This is a
signal that appears at the surface
of the material when using a dual
element delay line probe.
Measuring
extreme curva-
tures or areas of
restricted
access
Higher frequency trans-
ducers with smaller
diameters are required.
The smallest diameter
uses 3/16" crystals with
a contact area of .250"
Material being
measured
Mode
Transducer type
required
Notes
