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MAX II Bolt Tension Monitor
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.
5.4 Measurement Modes
In this section we will discuss the different measurements modes the MAX II is
capable of operating in, the transducers required, and the reasons for using specific
modes:
Pulse-Echo Mode (P-E)
Pulse-echo mode measures from the initial pulse (sometimes referred to as an
artificial zero) to the first echo (reflection). This is the standard most common
measurement mode typically used.
In this mode errors result from surface coatings applied to the bolt, as well as
temperature variations.
Since pulse-echo only requires one reflection (return-echo), it can be used for very
long bolts where achieving a second return-echo is difficult.
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