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mvip@mitech-ndt.com
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
phenomenons rest solely with the user of the instrument.
