Operating Manual

Chapter 3: Transducer Selection
Guide to Ultrasonic Inspection of Fasteners Page 15
Chapter 3: Transducer Selection
A wide variety of ultrasonic transducers are available.
Suitability for a specific application is determined based
on the transducer’s center frequency, diameter, and
damping. However, because there is often a broad range
of applications for which transducers are suitable, and
these ranges often overlap, it can be difficult to pick the
“best” transducer for a specific job.
NOTE: It is a generally accepted practice that the
same style and model probe be used when taking non-
tensioned (L-Ref) and tensioned-fastener measure-
ments of a fastener group. Further, it is preferable
that the same probe be used to make tensioned and
non-tensioned measurements of a fastener group.
3.1 General Acceptability
There is no single rule of thumb to follow when selecting
a transducer for a specific application. For some fasten-
ing systems, many different types of transducers will
measure with acceptable results. In the case of a hard-
to-inspect fastener, transducer selection becomes more
critical. The best way to evaluate an application is to use
the Bolt Mike’s waveform display and an assortment of
transducers. Try making readings on a fastener that’s
similar or identical to the ones you’ll be inspecting. Use
several different transducers and observe the waveform
display and the stability of the reading produced with
each transducer. While you’re using a transducer, ob-
serve the effects of removing and replacing it. Select
the transducer that provides a large-amplitude signal and
stable, repeatable readings.
3.2 Transducer Frequency
A transducer’s frequency rating refers to the resonant
frequency of the piezoelectric crystal. This is determined
by the thickness of the crystal material. A thin crystal
has a higher resonant frequency than a thick crystal.
The BoltMike will work with transducers in the 1 to 15
MHz (megahertz) range.
The frequency of the transducer affects the transmis-
sion of ultrasound in two different ways, beamwidth and
absorption. The
beamwidth (also referred to as directiv-
ity ) identifies how dispersed the shock wave becomes
as it travels over a specific distance. Beamwidth de-
creases (that is, the wave becomes more tightly focused)
as transducer frequency increases. This means that a
10 MHz transducer has a tighter beam (with a lower
beamwidth) than a 5 MHz version of the same transducer.
A tightly focused beam is desirable since it allows more
energy to reach the end of the fastener, making the noise
that reflects off the thread and shank areas less of an
issue.
However, as frequency increases, the absorption of the
ultrasound by the material also increases. Absorption
refers to the material’s ability to absorb (rather than re-
flect) ultrasonic sound energy. It interferes with the
shockwave, reducing the received signal’s resolution.
Lower-frequency ultrasound travels around small flaws
or air bubbles in the fastener’s without significant inter-
ference to the shock wave. Absorption is an especially
significant problem when inspecting more granular ma-
terial such as is found in castings.
In conclusion, lower transducer frequencies are better
suited as fastener lengths increase.
3.3 Transducer Diameter
A transducer’s rated diameter actually refers to the di-
ameter of its crystal. A transducer’s diameter affects the
efficiently with which it transmits sound as well as the
beamwidth of the transmitted ultrasound. Remember,
beamwidth identifies how dispersed the shock wave be-
comes as it travels over a specific distance. Beamwidth
decreases (that is, the wave becomes more tightly fo-
cused) and transmitting efficiency increases as the di-
ameter of the transducer’s crystal increases. Again, a
tightly focused beam is desirable since it allows more
energy to reach the end of the fastener, making the noise
that reflects off the thread and shank areas less of an
issue.
It’s generally preferable to select the largest-diameter
transducer available that will still fit on the fastener to be
measured. Note that external diameter of a transducer
equipped with a built-in magnet is much larger than the
piezoelectric crystal size. For example, a 1/4 inch 5 MHz
non-magnetic transducer has a case with a 3/8-inch
outside diameter. However, when a transducer with the
same 1/4-inch crystal is mounted in a magnetic housing,
the transducer’s outside diameter is 3/4 inch.
Purpose of Instrument and Transducer Zeroing
The BoltMike’s zeroing procedure occurs whenever the
user presses the Inst Zero key and follows the steps as
prompted. The procedure compensates for the actual
delay that occurs while the transmitted pulse travels
through the instrument’s circuitry, the probe cable, and
the probe’s head and contact surface. Variations in dif-
ferent probes and cables, as well as changes in the trans-