GE SpotChecker, Руководство по эксплуатации

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varying alloying constituents, this condition is mostly
ful
fi lled: the variation in sound velocity is so slight that
it is only of importance for high-precision measure-
ments. In other 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’s material
If the test object’s material is not homogeneous, the
sound may propagate at different sound velocities in
different parts of the test objects. In this case, an av-
erage sound velocity should be taken into account for
the range calibration. This is achieved by means of a
reference block whose sound velocity corresponds to
the average sound velocity of the test object.
If substantial sound velocity variations are to be ex-
pected, then the instrument calibration should be re-
adjusted to the actual sound velocity values at shorter
time intervals. Failure to do so may lead to false thick-
ness readings.
Effect of temperature variations
The sound velocity within the test object also var-
ies 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 and is then used on a warm or hot test object.
Such measurement errors can be avoided either by
warming the reference block to the same temperature
before calibrating, or by using a correction factor ob-
tained from 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.
Introduction
Important information on ultrasonic testing
12
Issue 02, 03/2009 SpotChecker