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the result. Since the real sound velocity in materials can be different than published values, the best results
a
re obtained when the instrument is calibrated with a sample block of the same material as the specimen.
This block must be flat (flat-parallel faces, with a smooth surface and thickness equal to the maximum
thickness which can be inspected on the specimen.
The operator must also be aware that the sound velocity may not be constant on the entire specimen due to
effects caused by heat treatments. This aspect needs to be taken into account when the results of thickness
measurement tests are evaluated. The calibration should always be checked after tests to minimize errors.
9.4.2 Procedure for the probe zero
The transducer calibration procedure must be performed as per the reference standard. The calibration block
must be clean, in good conditions and free from wear and/or oxidation: it must have a valid EN12223
certificate. Incorrect performance of the calibration and probe zero procedures or a non-conforming
calibration block cause inaccurate thickness readings and flaw indications.
9.4.3 Calibration for measuring flaws
When flaw measurements are performed, it is important to note that the amplitude of the indications is not
only related to the dimension of the discontinuity; even the flaw depth under the surface of the specimen
affects the amplitude due to the ultrasonic beam spread characteristics and field area in proximity to the
transducer. In addition, the discontinuity characteristics such as the orientation and type can alter the
response amplitude. For these reasons, the calibration should be performed with a sample block of the same
material as the specimen, with artificial discontinuities within the dimension and depth range of the material
to be tested. The operator is once again advised to consult specific books on ultrasonic testing for more
detailed information on the subject, which are not part of the scope of this manual.
9.4.4 Effects of temperature on calibration
The velocity of sound in the specimen and transducer wear vary when the temperature varies. All
calibrations must be performed on a sample block at the same temperature as the specimen, to minimize
errors.
9.4.5 Transducer conditions
The transducer used for the tests must be in good conditions and free from wear on the emitting surfaces.
The specific range of the transducer must include the complete range of thicknesses to test and/ or the types
of flaws to look for. The temperature of the material to test must be within the specific temperature range of
the transducer.
9.4.6 Using the couplant
Operators must be familiar with use of ultrasonic couplants. Skill must be developed in performing the tests,
so that the couplant is used and applied correctly, so that variations in the coupling thickness which can
cause errors or inaccurate readings are eliminated. The calibration and performance of the tests must be
carried out in similar coupling conditions, using the minimum quantity of couplant and applying an adequate
pressure on the transducer.
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