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Introduction
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 and is
then used on a warm or hot test object. Such measure-
ment errors can be avoided either by warming the refer-
ence block to the same temperature before calibrating,
or by using a correction factor obtained 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.
1-6
Ultrasonic evaluation of flaws
In present-day test practice, there are basically two
different methods of flaw evaluation:
If the diameter of the sound beam is smaller than the
extent of the flaw, then the beam can be used to ex-
plore the boundaries of the flaw and thus determine its
area.
If, however, the diameter of the sound beam is larger
than the size of the flaw, the maximum echo response
from the flaw must be compared with the maximum
echo response from an artificial flaw provided for com-
parison purposes.
Flaw boundary method
The smaller the diameter of the probe's sound beam,
the more accurately the boundaries (and therefore the
flaw area) can be determined by the flaw boundary
method. If, however, the sound beam is relatively
broad, the flaw area determined can substantially differ
from the actual flaw area. Care should therefore be
taken to select a probe which will give a sufficiently
narrow beam at the position of the flaw.
Issue 01, 04/2005
Important information on ultrasonic testing
Krautkramer USM 35X
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