Checkline TI-MINIMAX - Manual (Page 40)

Dakota Ultrasonics
34
Chapter 6 Bolt Preparation
The best balance between maximum frequency and noise suppression requires
selecting the best transducer for bolt measurement. The diameter of the
transducer (which is generally specified by the diameter of the actual
piezoelectric crystal) directly effects energy transmission: Larger diameter
crystals have greater ability to send and receive energy, and less of the energy
tends to spread laterally. The MINI-MAX Bolt Tension Monitor strives to achieve
this ideal balance: direct transmission of the strongest possible pulse, with the
least amount of noise and distortion, down and back the center of the bolt to
obtain the most accurate measurement.
6.1 Use of ultrasonic couplant
Sonic energy of the frequency used by the MINI-MAX 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 / air boundary at the end of the bolt, which causes most of the
energy to be returned as an echo. To make a familiar comparison: light crossing
the transparent glass boundary of a mirror and reflecting back an image from the
nontransparent silvered back is much like an echo.
Any air gap between the face of the transducer and the end of the bolt will
prevent the sonic energy from crossing. 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 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 shipped with the MINI-MAX, produce 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 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.
Again, the ultrasonic couplant provided with the MINI-MAX works best. Apply
only the smallest quantity of couplant required to fill the air gap, and carefully
seat the transducer so that any trapped air or excess couplant is squeezed out of
the interface.