Couplants
All ultrasonic applications require some medium to couple the sound from the transducer to the test piece.
Typically a high viscosity liquid is used as the medium. The sound used in ultrasonic thickness measurement does not
travel through air efficiently.
A wide variety of couplant materials may be used in ultrasonic gauging. Propylene glycol is suitable for most
applications. In difficult applications where maximum transfer of sound energy is required, glycerin is recommended.
However, on some metals glycerin can promote corrosion by means of water absorption and thus may be undesirable.
Other suitable couplants for measurements at normal temperatures may include water, various oils and greases,
gels, and silicone fluids. Measurements at elevated temperatures will require specially formulated high temperature
couplants.
Inherent in ultrasonic thickness measurement is the possibility that the instrument will use the second rather
than the first echo from the back surface of the material being measured while in standard pulse‐echo mode. This may
result in a thickness reading that is TWICE what it should be. The Responsibility for proper use of the instrument and
recognition of these types of phenomenon rests solely with the user of the instrument.
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