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greater ability to send and receive energy, and less of the energy tends to spread laterally. Your gauge is
designed to achieve this ideal balance; direct transmission of the strongest possible pulse, with the least
amount of noise and distortion, down and back the centreline of the bolt to obtain the most accurate
measurement.
5 BOLT PREPARATION
5.1 CORRECT USE OF ULTRASONIC COUPLANT
Sonic energy of the frequency used by this gauge travels well through solid materials and most liquids. It
does not travel well through air. This variable resistance to the passage of sonic energy is called sonic
impedance. It is the sudden change in impedance which occurs when the sound pulse attempts to cross
the metal to air boundary at the end of the bolt which causes most of the energy to be returned as an echo,
much like light reflected by the silvered back surface of a mirror.
Any air gap between the face of the transducer and the end of the bolt will prevent the sonic energy from
being transmitted into the bolt. The gap must be filled with a suitable coupling fluid. Normally, a liquid
ultrasonic couplant is applied between the transducer and the bolt. Since liquid has a sonic impedance
closer to the transducer and bolt material than air, it forms a continuous path for the outgoing pulse and the
returning echo.
Many liquids can serve as an adequate couplant, however liquids with lower sonic attenuation will produce
better results. Liquids containing glycerin offer such low impedance, and couplant manufactured for
ultrasonic testing, such as the one supplied with this gauge, produces the best results.
The single purpose of the couplant is to fill the air gap between the transducer and the bolt end. This is
accomplished more easily with a liquid that is viscous enough to stay in place. Very viscous substances
however can create such a thick layer between the transducer and bolt that measurement errors occur.
Also, viscous substances can trap air bubbles, which prevent adequate energy transmission.
The ultrasonic couplant provided with your gauge works best. Apply only the smallest quantity of couplant
required to fill the air gap, and carefully place the transducer so that any trapped air or excess couplant is
squeezed out of the interface.
5.2 CONTACT BETWEEN THE TRANSDUCER AND THE BOLT
The goal is to transmit as much sonic energy as
possible from the transducer into the bolt, and to
send that energy, to the greatest extent possible,
down the centre of the bolt and back.
In order to transmit sonic energy straight along the
axis of the bolt, the transmitting end of the bolt must be smooth, flat and perpendicular to the axis of the bolt
and must seat the entire active surface of the transducer with minimum gap. The end of the bolt may need
to be cleaned, ground, etc. to achieve the required surface. Typical problems with the transducer contact
end of the bolt are listed below:
Rough or irregular surfaces.
This condition prevents adequate
contact with the transducer. Couplant will fill the irregularities in the
surface, but energy transmission will be reduced and dispersed causing
mode conversions to occur from reflections off the side walls of the bolt.
Bolt ends not perpendicular to the axis of the bolt.
Energy will be
transmitted toward the side wall and reflect along the bolt, yielding poor
signal quality and possible measurement errors. Avoid alignment errors
exceeding 2 degrees.
Rusted, dirty, or thick paint-covered bolt ends.
These coatings
prevent sonic energy from travelling between the transducer and the
bolt. Very thin coating or plating is acceptable.
