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Coupling medium
Every ultrasonic application requires some medium to couple the sound from the trans-
ducer to the tested material. Typically, a high viscosity liquid is used as the medium.
The sound used in ultrasonic thickness measurement doesn't travel through air effi-
ciently.
A wide variety of coupling mediums may be used. Propylene glycol is suitable for
mostly all applications. In difficult applications, where a maximum transfer of sound
energy is required, glycerine is recommended. However, on some metals glycerine
may promote corrosion by means of water absorption, which is undesirable.
Other suitable coupling medium for measurements at normal temperatures may in-
clude water, various oils and greases, gels and silicone fluids. Measurements at ele-
vated temperatures will require specially formulated high temperature coupling me-
dium.
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 being in standard pulse-echo mode.
This may result in a thickness reading that is TWICE what it should be.
The responsibility of a proper use of the instrument and the recognition of these types
of phenomenon solely rest with the user of the instrument.
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