Operating Manual

Transducer Selection
Basic to the operation of the BoltMike III is the ultrasonic transducer. There are a variety of transducers available, and
there is overlap in their applications. This can make choosing the “best” transducer for the job difcult. Transducers vary in
center frequency, diameter, and damping. There is no “rule of thumb” for selection. For an “easy” fastener, a large variety of
transducers will measure with great results. In the case of a difcult fastener, transducer selection becomes more critical.
The best way to evaluate an application is to use the BoltMike III waveform display. Try making readings with an assortment
of transducers and observe the waveform display and the stability of the reading. Pick a transducer that provides a large
signal and stable, repeatable readings when removing and replacing the transducer.
Transducer Frequency
The frequency of a transducer refers to the resonant frequency of the piezoelectric crystal. This is determined by the thick-
ness of the crystal material. A thin crystal has a higher resonant frequency than a thick crystal. The BoltMike III will work
with transducers in the 1 to 15 megahertz range.
The frequency of the transducer effects the transmission of ultrasound in two different ways, beam width and absorption.
The beam width or directivity of the sound decreases as frequency increases. This means that a 10MHz/.25” transducer
has a tighter beam than a 5Mhz/.25” transducer. It is desirable to have a closely focused beam since more energy
reaches the end of the fastener and noise from reections off the thread and shank areas is reduced.
However, as frequency increases, the absorption of the ultrasound by the material also increases. This is especially true
of the granular material found in castings. The lower frequency ultrasound travels around small aws or air bubbles in the
material without reecting.
Transducer Diameter
The diameter of the transducer effects the beam width and directivity of the ultrasound. Also, a larger diameter crystal is
more efcient at transmitting sound. The larger the diameter of the crystal element the more directed the ultrasound
becomes. It is desirable to have a closely focused beam since more energy reaches the end of the fastener and noise from
reections off the thread and shank areas is reduced.
Therefore, select the largest transducer diameter that will t on the fastener. Transducers that have a magnet built in are
signicantly larger than the piezoelectric element size. For example, the outside diameter of the 5MHz/.25” non-magnetic
transducer is 3/8 inch while the same transducer in a magnetic housing has an outside diameter of 3/4 inch.
Contact Information
StressTel
50 Industrial Park Road
Lewistown, PA 17044
Tel: [phone]
Tel: [phone]
E-mail: [e-mail]
www.stresstel.com
3