Dakota Ultrasonics MINI--MAX - Manual (Page 40)

Dakota Ultrasonics
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5.2 Features of the MINI-MAX
The Dakota Ultrasonics MINI-MAX, Ultrasonic Bolt Tension Monitor, defines the
State of the Art in the measurement of the actual clamp load produced by
tightening a fastener. The MINI-MAX can measure time, elongation, load, stress,
or %strain in bolts of virtually any material from 1 inch to 8 feet in length,
depending on the Mini-Max version. By storing the reference waveform and
displaying it for comparison while the elongation is being measured, the MINI-
MAX minimizes operator training.
5.3 Ultrasonic waves
Ultrasonic measurement requires the transmission of a suitable quantity of
ultrasonic energy through the length of the bolt. The relationship of the energy
pulse frequency to its penetration is important in energy transmission. Lower
frequencies produce longer wavelengths that will travel further through a given
substance: while higher frequencies produce shorter wavelengths. To use a
familiar example: AM radio signals are broadcast at relatively low frequencies
and can be received hundreds of miles away, over the horizon. Higher quality FM
radio and television signals are broadcast at much higher frequencies, and can
only be received within a comparatively short line-of-sight or distance.
The same phenomenon exists with ultrasound. A low frequency 1 MHz pulse
travels much farther through metal than a 5 MHz pulse. Therefore, a lower
frequency transducer is able to achieve an echo in a longer bolt, or in a bolt
made of metal with higher resistance to sound transmission (attenuation). While
the lower frequency has more penetration power, it also produces more
unwanted noise. Low frequency energy tends to spread, much like an
unfocussed beam of light. When low frequency energy is introduced at the end of
a bolt, a significant portion is bounced from side to side within the cylindrical
shape, producing a noisy and distorted echo. Higher frequency pulses tend to
travel more directly down and back the centerline of a bolt, with less noise and
distortion.