KERN & SOHN TN 60-0.01EE - Manual (Page 15)

TN_EE-BA-e-1410 15
these. In echo-echo mode, the paint/ coating thickness will be eliminated entirely and
the steel will be the only material measured.
Suitability of materials
Ultrasonic thickness measurements rely on passing a sound wave through the mate-
rial 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 trans-
ducer 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 efficient-
ly.
A wide variety of couplant materials may be used in ultrasonic gauging. Propylene
glycol is suitable for most applications. In difficult applications where maximum trans-
fer 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 unde-
sirable. Other suitable couplants 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 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 in-
strument.