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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.
