GE 37 400, Technical Reference And Operating Manual

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1 Introduction Important information on ultrasonic testing
1-6 Issue 2 (12/2013) USM 36
of importance for high-precision measurements. In oth-
er materials, e.g. nonferrous metals or plastics, the
sound velocity variations may be even larger and thus
affect the measuring accuracy.
Effect of the test object material
If the material of the test object is not homogeneous, the
sound waves may propagate at different velocities in dif-
ferent parts of the test object. An average sound velocity
should then be taken into account for the range calibra-
tion. This is achieved by using a reference block with a
sound velocity equal to the average sound velocity of
the test object.
If substantial sound velocity variations are expected,
then the instrument calibration should be adjusted to the
actual sound velocity values at shorter time intervals.
Failure to do so may lead to false thickness readings.
Effect of temperature variations
The sound velocity within the test object also varies as
a function of the material's temperature. This can cause
appreciable errors in measurements if the instrument
has been calibrated on a cold reference block, whereas
the measurement is carried out on a warm test object.
Such measurement errors can be avoided either by ad-
justing the temperature of the reference block used for
calibration or by taking the temperature effect into con-
sideration on the basis of a correction factor obtained
from published tables.
Measurement of remaining wall thickness
The measurement of the remaining wall thickness on
plant components, e.g. pipes, tanks, and reaction ves-
sels of all types which are corroded or eroded from the
inside, requires a perfectly suitable gauge and special
care in handling the probe.
The inspectors should always be informed about the
corresponding nominal wall thicknesses and the likely
amount of wall thickness losses.