pockets will cause an early reflection of the ultrasound beam. This effect will be noticed as a sudden
decrease in thickness in an otherwise regular surface. While this may impede accurate measurement of
total material thickness, it does provide the user with positive indication of air gaps in the laminate.
Suitability
of
materials
Ultrasonic thickness measurements rely on passing a sound wave through the material being measured.
Not all materials are good at transmitting sound. Ultrasonic thickness measurement is practical in a
wide variety of materials including metals, plastics, and glass. Materials that are difficult include some
cast materials, concrete, wood, fiberglass, and some rubber.
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.
