FYI: Reference and Calibration Standards
To ensure accurate and repeatable inspection, ultrasonic testing
equipment must be standardized and calibrated so that data
taken by different operators are comparable and can be
matched against inspection norms. This is accomplished through
the use of reference and calibration standards or test blocks.
Selecting a Reference Standard. Selection of a reference standard is
determined by testing technique, type of material to be inspected and
form,type of discontinuity to be detected, and specification requirements.
Reference standards, such as those designed to ASTM,ASME or AWS
specifications, are used to standardize equipment responses.Blocks with
flat bottom holes (Fig. 1) are often used to standardize amplitude of the
detected signal with respect to the effective area or distance of known
reflectors.Area (amplitude or distance) amplitude curves are usually
constructed using such blocks. In some cases, blocks with side drilled
holes are also used for such standardizations.Reference standards varying
acoustically from the test object by more than 6 decibels are usually
considered unacceptable for use.
Preparing a Reference Standard. Sometimes, it is preferable or
required to prepare a reference standard from a piece of the same
material as that to be tested, by introducing notches or holes into a
sample or into the actual test object. The advantage of such a reference
standard is that the test object and the standard will have the same
composition,manufacturing history, surface condition and geometry.
the disadvantages are that usually there will be fewer artificial reflectors
and it may not be possible to manufacture the reflectors as accurately as
might be done with a separate standard.
Other types of standard test blocks (and some of the above reference
blocks) are used to calibrate the ultrasonic testing equipment with
respect to essential variables such as sweep length, pulse energy and
amplification, search unit characteristics, sensitivity, resolution and
linearity.Typical calibration blocks include IIW type blocks,DC, SC,
DSC and MAB blocks.Two uses of the IIW type blocks are shown in
Fig. 2a and b.
Conclusion. In all cases, reference standards must be prepared and
used in strict accordance with well designed specifications that cover the
material, the fabrication and the application of the blocks.
From ASNT Level II Study Guide: Ultrasonic Testing Method. Reprinted with permission of The American Society for Nondestructive Testing, Inc. This reprint contains copyrighted property of The American Society for Nondestructive Testing, Inc. and may not be duplicated or altered in any manner.