Olympus 45MG, User Manual

Page: 73 / 246

DMTA-10022-01EN, Rev. C, January 2015
Basic Operation 63
• Velocity variations:
An ultrasonic thickness measurement is accurate only to the degree that
material sound velocity is consistent with the instrument calibration. Some
materials exhibit significant variations in sound velocity from point to point.
This happens in certain cast metals due to the changes in grain structure that
result from varied cooling rates, and the anisotropy of sound velocity with
respect to grain structure. Fiberglass can show localized velocity variations
due to changes in the resin/fiber ratio. Many plastics and rubbers show a
rapid change in sound velocity with temperature, and as such, velocity
calibration must be performed at the temperature of the location where the
measurements are made.
Phase reversal or phase distortion
The phase or polarity of a returning echo is determined by the relative acoustic
impedances (density × velocity) of the boundary materials. The 45MG performs
computation based on the customary situation, where the test piece is backed by
air or a liquid, both of which have a lower acoustic impedance than metals,
ceramics, or plastics. However, in some specialized cases, such as measurement of
glass or plastic liners over metal, or copper cladding over steel, this impedance
relationship is reversed, and the echo appears phase-reversed. In such cases, it is
necessary to change the appropriate echo detection polarity in order to maintain
accuracy (see “Detection of Echo 1 and Echo 2” on page 179). A more complex
situation can occur in anisotropic or inhomogeneous materials, such as
coarse-grain metal castings or certain composites, where material conditions
result in the existence of multiple sound paths within the beam area. In such
cases, phase distortion can create an echo that is neither cleanly positive nor
negative. Careful experimentation with reference standards is necessary in such
cases in order to determine the effects on measurement accuracy.
5.3 Measuring Thicknesses
You can start performing thickness measuring as soon as a transducer is connected
(see “Setting Up the Transducer” on page 51) and the instrument is calibrated (see
“Calibrating the Instrument” on page 55).
To measure thicknesses
1. Apply couplant to the measurement spot on the test block or tested part.