Calibration of Ultrasonic Units
The measurement of any parameter is of no value without some certainty that the measurement is of sufficient accuracy. This NDT Solution paper provides useful insight to ultrasonic calibration. Understanding of the process can ensure that you can specify and obtain meaningful calibration and, ultimately, reliable ultrasonic measurements.
G.P. Singh
Contributing Editor
Background
The normal practice in most test methods is the calibration of the equipment to ensure the accuracy and repeatability of the test. Industry standards are usually invoked to give guidance regarding the calibration and adjustment of testing equipment. The document MIL STD 45662A: Military Standard for Calibration System Requirements, has stood alone for many years as the means by which a calibration system should be set up and applied. New standards have now emerged to replace MIL STD 45662A. Two such documents are International Standard ISO 10012-1: Quality Assurance Requirements for Measuring Equipment (metrological confirmation system for measuring equipment), and American National Standard ANSI/NCSL Z540-1: Calibration Laboratories and Measuring and Test Equipment - General Requirements.These documents provide, to some degree, guidance for selecting intervals for the frequency of calibrations or instrument checks, personnel, traceability, reference materials, environment, procedures, and records. These documents, however, do not give any direction on how to calibrate specific equipment. They simply state that some interval be selected and that some standard for calibration be established based on usage and stability, with a specified measurement uncertainty and accuracy. To specify to a calibration laboratory that an ultrasonic instrument, for example, be calibrated in accordance with the requirements of MIL STD 45662A is totally missing the intent of the document. Any calibration laboratory that provides calibration based on this requirement does not understand the calibration requirements or they may simply invoke what they believe to be normal calibration limits (at times this is stated as manufacturer's standards) or their own values that they have established.
This area is somewhat ambiguous for the normal quality control person in charge of calibration.
The actual guidance for calibration is normally stated in the controlling specification or standard which specifies ultrasonic inspection. The controlling document usually indicates the calibration tolerances or limits as well as the calibration frequency. Typical examples of these controlling documents are MIL STD 2154, AWS D1.1, ASME Section V, Articles 4 and 5, as well as several ASTM test methods, standards, or practices. These standards specify the extent to which calibration is required. They may require only vertical and horizontal linearity, or they could be quite extensive and require numerous tests such as amplitude (dB) linearity and dynamic range, near and far surface resolution, vertical and horizontal limits as well as linearity, signal to noise ratio, and sensitivity.
The extent to which calibration is taken seriously, or is even understood, depends upon the experience and dependability of the person or company performing the checks or tests on the instrument. The answers to the following questions can affect the reliability of the calibration.
- Does the user have the proper calibration equipment and/or test blocks?
- Is ultrasonic testing a regular requirement or is it performed infrequently?
- Does the user have an established calibration program?
- Does the user understand why the calibrations are required?
- Does the user know what calibrations and methods to specify, if performed by an outside source?
- Does the calibration source or facility, if an outside organization, follow the customer's requirements?
- Does the outside organization know the manufacturer's specifications or have the equipment manuals
- Does the outside organization have NIST traceability?
Calibration Mystery
One area that causes confusion is the definition of "acceptable" calibration. If an ultrasonic instrument is sent out for calibration for conformance to ASTM E317 and an electronic evaluation in accordance with ASTM E1324 is made, does this mean that the instrument is calibrated and acceptable for the work to be performed? Or, what if the instrument is sent out with the statement "calibrate ultrasonic unit" and an electronic or manufacturer's calibration is performed? This is an area that is somewhat ambiguous for the normal quality control person in charge of calibration, or even the ultrasonic technician who may have to evaluate the results of the calibration or checks made on the instrument by an outside calibration laboratory.
Figure 1 - Basic configuration for a resolution block, from ASTM E317. Reprinted with premission of ASTM.
Basically, two different methods exist for calibration. The first method is the use of electronic calibration or manufacturer's specification to ensure that the ultrasonic unit is operating within the specifications developed by the instrument manufacturer. The second method is the evaluation of the performance characteristics of the entire ultrasonic system. The two methods of calibration are actually designed to work together as complements of one another. The scope of ASTM E1324 describes the intent of the electronic calibration and indicates that it is to be used in conjunction with ASTM E317.
Electronic Calibration
The electronic calibration is used for several different reasons. One reason is to verify that the instrument is working properly within the manufacturerÕs specifications for each electrical circuit in the system. These are measured against an electrical load or signal by the use of an oscilloscope, multimeter, or other electronic device. Calibrated external attenuators are also used to drive the applicable sections of the circuits and provide specific input. In any case, the limits or range of acceptable values must be specified.The second reason an electronic calibration might be used is to compare the response between two different ultrasonic instruments. This could be required when attempting to duplicate test results or when comparing instruments to determine which unit would produce the best response in a specific test situation.
Areas that are examined electronically are gain, linearity, pulse amplitude, time of flight peak mode, gate triggering, noise level, cross talk, pulser timing, pulse power, pulse duration, damping switch and filtering, time corrected gain, voltage input and output, as well as a host of other characteristics.
System Calibration
The system calibration, in accordance with ASTM E317, is used to verify the performance characteristics of the ultrasonic unit as used in conjunction with a coupling medium, transducers, coaxial cables and connectors, and test pieces or test blocks that are acoustically similar (heat treat condition, grain size, grain flow, surface finish) to the material to be tested. The scope of ASTM E317 also indicates that performance limits must be specified. These are found in the specification requiring ultrasonic inspection.The calibration checks of vertical and horizontal limits and linearity are fairly straightforward in application as well as the accuracy of gain controls (Birks et al., 1991). The near and far surface resolution, however, require that the customer specification give direction as to what the minimum limits are.
No standard test blocks exist for every condition that may be specified, so special blocks may need to be manufactured to verify the resolution capabilities of the system. For instance, MIL STD 2154 requires that the entry surface resolution is 3.175 mm (0.125 in.) for materials other than forgings for thicknesses up to 31.75 mm (1.25 in.). This requires a 0.4 mm (0.015 in.) flat bottom hole at a 3.175 mm (0.125 in.) metal path. Many aerospace parts manufactured today are designed with a minimum machining envelope to save money on expensive materials such as certain stainless steels, titanium, Inconels, Hastelloys, and other exotic materials. At times the customer requirement may be as little as 1.3 or 1.5 mm (0.05 or 0.06 in.) near surface resolution. The use of test blocks specified in MIL STD 2154 for calibration could completely miss the requirement for near surface resolution. If an outside source is used and only an electronic calibration is performed, the requirement could not be verified. In addition, if external calibration is performed, the specific limits would need to be delineated with regard to surface resolution requirements.
Many ultrasonic units used for field testing do not have the response necessary to distinguish near or far surface resolution in some materials used in industry. Many high temperature materials are highly attenuative, and noise levels could be too high for many instruments to resolve. Figure 1, from ASTM E317, provides the basic configuration for a resolution block. The hole diameters, metal travel distances, and surface finish requirements must be specified by the user.
Calibration Verification
When calibration is performed by an outside source, the equipment used for calibration should itself be calibrated at specific intervals and should be traceable to the National Institute of Standards and Technology, or NIST (formerly the National Bureau of Standards, or NBS) if a standard exists. Otherwise, the traceability should be to a recognized industry standard.Proper evaluation of a calibration source requires more than a statement that indicates the instrument was calibrated to NIST. The NIST traceability numbers should be listed or should be available for each apparatus used in calibration. The NIST number should also have a date indicating the actual date the instrument was compared against the NIST standard. A master instrument or standard is typically recalibrated every one to five years, based on the type of instrument, its stability and the amount of use it receives, and the original equipment manufacturer's recommendations. Care should be exercised in this review, since it has been found that some sources have used NBS traceability numbers for up to twenty years without performing recalibration. In any case, the calibration source should have a list of the frequencies at which masters are sent for comparison to NIST standards.
Another situation that can lead to inaccuracies is when a calibration source uses a NIST number and its instrument may not have been directly compared to a NIST standard. In other words, the standard to which the calibration source compared its equipment may have been compared to another instrument that was compared to a NIST standard. This "chain" between the master instrument and the NIST standard can become quite long at times, with three or four "links." This tends to allow for inaccuracies in the calibration, since the deviations may build up. When a number of links are used, each link should identify the amount of uncertainty that is inherent in the calibration process. Typically, a five to one ratio is used: the outside organization must have its equipment calibrated to an accuracy of two percent in order to provide an accuracy of within ten percent to its customers.
Conclusion
The calibration of ultrasonic instruments and the standards used for the calibration both require a detailed review of the applicable specifications invoked by customers. An understanding of the different means of calibration is important because the use of the wrong calibration requirements could invalidate many hours of inspection work. Electronic calibration should be used in conjunction with a system performance evaluation, since the two methods complement each other. One logical means to accomplish calibration is to conduct the performance based calibrations on a set interval and augment these with the electronic calibration on a semi-annual, annual, or other basis as determined by equipment use, customer requirements, and stability. In any case, the performance based calibration should take precedence.Many of the requirements for ultrasonic equipment calibration were based on the response available from older instruments. The values listed in many documents are no longer valid since newer units can easily pass the requirements. Industry on a large scale is moving toward performance based standards. ISO documents are rapidly being developed and are taking the place of outdated standards that have been around for many years. Input from equipment manufacturers, companies specifying ultrasonics, and users is needed to provide industry with viable standards that will be practical for use with the newer equipment now available. Some manufacturers have already introduced computer controlled calibration checks that electronically drive the instrument and provide a report with the calibration values and limits listed. This is a giant stride toward simplifying and standardizing the calibration checks. A similar consolidation of methods and requirements is needed in the performance based calibration area. Of course, each industry has particular needs, and some may not want strict limits that could increase the costs of calibrations without providing significant benefit to the test. The education of technicians in the area of equipment calibration could also help to provide calibration laboratories with the correct information and proper direction needed for satisfactory calibration.
References
ASTM E1324: Standard Guide for Measuring Some Electronic Characteristics of Ultrasonic Examination Instruments.ASTM E317: Standard Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Testing Systems.
Birks, Al, Robert E. Green, Jr., and Paul McIntire, Nondestructive Testing Handbook, Volume 7, Ultrasonic Testing, 2nd ed., 1991, pp 465-470. ASNT, Columbus, OH.
Copyright © 1996 by the American Society for Nondestructive Testing, Inc. All rights reserved.
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