Olympus 27MG, User Manual

Page: 71 / 98

DMTA-10043-01EN, Rev. C, July 2016
Application Notes 63
• Velocity Variations
A number of materials exhibit significant variations in sound velocity from point-
to-point within the material. Certain types of cast stainless steels and brass exhibit
this effect due to a relatively large grain size and the anisotropy of sound velocity
with respect to grain orientation. Other materials show a rapid change in sound
velocity with variations in temperature. This is characteristic of plastic materials
where temperature must be controlled in order to obtain maximum precision in
the measurement.
• Sound Attenuation
Sound attenuation or absorption in many organic materials, such as low density
plastics and rubber, occurs when sound is attenuated very rapidly at the
frequencies used in normal ultrasonic thickness gaging. Therefore, the maximum
thickness that can be measured in these materials is often limited.
10.2 Transducer Selection
For any ultrasonic measurement system (transducer plus thickness gage), there is a
minimum material thickness below which valid measurements will not be possible.
Normally this minimum range will be specified in the manufacturer’s literature. As
transducer frequency increases, the minimum measurable thickness decreases. In
corrosion applications, where minimum remaining wall thickness is normally the
parameter to be measured, it is particularly important to be aware of the specified
range of the transducer being used. If a dual is used to measure a test piece that is
below its designed minimum range, the gage may detect invalid echoes and display
an incorrectly high thickness reading.
Table 6 on page 64 lists approximate minimum measurable thicknesses in steel for the
standard transducers used with the 27MG Ultrasonic Thickness Gage. Note that these
numbers are approximate. The exact measurable minimum in a given application
depends on material velocity, surface condition, temperature, and geometry, and it
should be determined experimentally by the user.