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Calibration for Nondestructive Testing

by Stuart Kleven*, Israel Vasquez^† and David Atkins^‡

 

INTRODUCTION

Standards are as old as human society itself. During the era of the pharaohs, calibration standards were used for measurement during the construction of the great pyramids at Giza. The Royal Cubit was the length from the back of the pharaoh's elbow to the tip of his middle finger. This was established in a stable material such as granite. Then working standards were made and compared to the master. These were issued to workers, and they were required to return the working standards for comparison against the Royal Cubit master every full moon. Failure to submit the working standard for verification was punishable by death. The accuracy obtained by the use of the working standards was phenomenal. In a structure measuring approximately 230.4 m (756 ft), the Egyptian builders were accurate to within 115 mm (4.5 in.). This is about a 0.05% accuracy (National Council of Standards Laboratories, 2006). Weights (Figure 1) from most ancient societies have been discovered and are accurate within tenths or hundredths of a gram when compared to the required standard weight. Like us, ancient people were very concerned with maintaining accuracy. Even within the Bible, it was stated that "the Lord detests differing weights, and dishonest scales do not please him" (Proverbs 20:23).

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Over the years, a number of calibration documents
have been specified for use.

Today, the calibration of testing equipment would appear to be a subject that is unimportant or secondary to their immediate purpose. Many companies merely send out equipment and indicate on purchasing documents that a calibration certification is required. This ignores the question of what a proper calibration is. One complicating factor is the profusion of different types of equipment and equipment manufacturers, which can also add confusion to the situation. For instance, some units are analog and others are digital. What rules apply? Can one type be used to verify the other? In addition, most governing specifications in nondestructive testing only give the frequency that is expected for the calibration of certain items, but rarely give any limits or tolerances. This can cause quite a quandary for the inexperienced and even the experienced quality professional. This paper will attempt to address many of the issues that have caused problems and make possible suggestions for standardizing some calibrations.

CALIBRATION DOCUMENTS

Over the years, a number of calibration documents have been specified for use in NDT. The old standby for many years was MIL-C-45662A. This was later converted to MIL-STD-45662 and eventually revised to MIL-STD-45662A. This stood for quite a number of years as the only guideline for calibration. Many auditors and customers simply said that it was up to the calibration lab to establish a system and then follow it based on the guidance of MIL-STD-45662A. In 1984, MIL-HDBK-52A was issued to provide guidance in applying MIL-STD-45662A. In 1989, MIL-HDBK-52B was released to further clarify the application of the military standard. Recent developments in the calibration area have produced a number of documents and schemes addressing calibration, both for the calibrating laboratory and for the company specifying calibration. These include ANSI Z540-1 (the US equivalent to ISO Guide 25), ISO 10012-1, and ISO 17025 (formerly ISO Guide 25).

Figure 1 — Ancient stone weights.

ISO 17025, General Requirements for the Competence of Testing and Calibration Laboratories, is the main document used today. It consists of the following components or characteristics:

CALIBRATION SOURCES

The first step in calibration is to find a dependable calibration source for performing the work. The typical approach is to determine who can supply the calibration at the lowest price. Taking this course could, however, prove to be a problem later. The first step to finding a dependable source is to look for some sort of nationally recognized acceptance (like accreditation). This means that someone from an independent, third-party source has assessed the calibration source and found it compliant to the standard or specification. While this does not necessarily ensure perfection, it removes quite a few factors that can affect the calibration, such as the use of adequate quality procedures, valid test methods, calibrated equipment with traceability to the National Institute of Standards and Technology (NIST), an understanding of the accuracy and uncertainty limits, and proper personnel training and qualification. Laboratories with accreditation to the National Voluntary Laboratory Accreditation Program (NVLAP), the American Association for Laboratory Accreditation (A2LA), the Laboratory Accreditation Bureau (LAB) or other accredited registrars are preferred.

Each of these accrediting bodies maintains a listing of the calibration laboratories that have been assessed. Assessment may be performed by the company requiring the calibration as well. This may be achieved by a desk audit (filling out a checklist), an onsite audit or by inspecting and reviewing the calibration certificate and the instrument upon receipt. Other factors can affect the selection of calibration laboratories as well. If a delicate or bulky equipment requires calibration, shipment across the country can cause problems. The length of time a piece of equipment is out for calibration may necessitate the acquisition of two units to maintain calibrated status. In addition, the handling in shipping can cause inherent troubles, possibly by damaging the unit during transit or by affecting the newly calibrated condition. The lack of personal contact and confidence is missing when the calibrating laboratory cannot be readily visited. Onsite audits are less likely to be performed when the laboratory is not in close proximity to the company using their services.

DEFINITIONS

It would be helpful to define terms with regard to calibration. The following definitions were obtained from the documents previously cited in this paper.

Calibration

The set of operations which establish, under specified conditions, the relationship between values indicated by a measuring instrument or measuring system, and the corresponding standard or known values derived from the standard. In simple terms, comparing a known against an unknown. (Not the same as standardization, which is sometimes called calibration. For example, radiographic film densitometers are standardized with a "working" step tablet, and calibrated with an NIST traceable "master" step tablet.) Calibration can refer to several different types of actions taken by a company: outside calibration of master equipment directly traceable to NIST; in-house calibration of equipment to an in-house master directly calibrated to NIST; in-house calibration of equipment by an outside source with master equipment traceable to NIST.

Accuracy

Conformity to a certified or approved standard. A measure of closeness of agreement between a measured result and the true value. Accuracy is a qualitative concept.

Uncertainty

The result of the evaluation aimed at characterizing the range within which the true value of a measurand is estimated to lie, generally within a given likelihood.

Under MIL-STD-45662, collective uncertainty shall not exceed 25% of the acceptable tolerance for the characteristic being calibrated. This is commonly called the 4:1 test accuracy ratio. Sources contributing to uncertainty include the reference standard or materials used, the method and equipment used, environmental conditions, the condition of the item being calibrated, and the technician performing the calibration.

For ISO 17025, the uncertainty must be developed and justified based on analysis and consideration of the conditions listed above. Some customer specifications have listed uncertainty requirements ranging from 4:1 to 10:1. Care should be exercised in agreeing to these limits, since they may be unattainable under certain situations.

Tolerance

The set of values for a measurand for which the error of a measuring instrument is intended to lie within specified limits (allowable deviation from a certified or approved standard, such as +/- 10%).

Stability

The ability of a measuring instrument to constantly maintain its metrological characteristics.

Drift

The slow variation with time of a metrological characteristic of a measuring instrument (especially applicable to electronic instrumentation).

Traceability

The ability to relate individual measurement results through an unbroken chain of calibrations to one of the following:

Adequacy of the Measurement Standard

The measurement standard shall be traceable and have the accuracy, stability, range and resolution required for the intended use.

Precision

This is the closeness of multiple readings to each other.

Bias

This is the accuracy of the closeness of multiple readings to the true value.

STANDARDS

Calibration standards are normally established in the US by NIST, formerly known as the National Bureau of Standards (NBS). Each country has their own respective agency. For a complete listing, visit NIST's Web site at www.nist.gov/oiaa/national.htm. Traceablility to these known standards is the normal method used for calibration. At times, NIST has established physical constants that are used for calibration, such as the speed of light in a vacuum, the Josephson frequency voltage ratio, and quantized Hall resistance. Consensus standards may be established where no national standards exist. These are standards that everyone agrees on as a standardized means for calibration. If none of the above exists, original equipment manufacturer (OEM) standards may be used.

FREQUENCY OF CALIBRATION

While some specifications or standards do establish a calibration frequency, most leave it up to the user. The natural inclination is to set the frequency as long as possible to reduce calibration costs. This of course may increase the likelihood that, should the instrument be received by the calibration source with a "significant out-of-tolerance" condition, all products affected by this instrument over the extended period of use is now suspect. The proper way to establish the correct frequency is to examine the literature from the equipment manufacturer to determine the stability of the unit. The next step is to set a shorter frequency based on the stability, purpose and usage of the equipment. Typically, electronic instruments tend to drift or get out of calibration sooner than fixed gages or measuring tools such as microthickness gages. To give an example, an electronic ohmmeter may be set at a quarterly frequency if it is used fairly often. Then, after four cycles where documentation demonstrates that the equipment has maintained its accuracy throughout the calibration intervals, the frequency could be extended to six months. This could be repeated for three or four cycles and again, if no change is noted, a slightly longer frequency could be established, such as annually. If an instrument comes back from calibration with an out-of-tolerance condition, it may be advisable to shorten the frequency for at least one cycle to determine whether or not the condition is due to inherent problems with the instrument or if it was an isolated incident. (See "Out-of-Tolerance Conditions," below, for more information.)

Several newer standards have determined ways to set the frequency. The following guidelines for the determination of confirmation intervals for measuring equipment are described in detail in ISO 10012-1, Annex A:

UNCERTAINTY

Uncertainty is the amount of deviation allowed within the tolerance. Based on the current ISO 17025, the uncertainty must be stated by the calibration laboratory. Each laboratory shall have and apply a procedure to estimate the uncertainty of measurement for all types of calibrations. They are required to develop an uncertainty budget based on internal factors, such as identifying all the components of uncertainty and making the best possible estimation, and ensuring that the form of reporting does not give an exaggerated impression of accuracy. These factors include environment, physical constants, the measuring procedure, definition of the measurand, the measuring object, the metrologist or technician, software and calculations, measurement setup, the actual measuring equipment, and the reference element. Once each factor is identified as providing uncertainty, the amount must be determined to the best possible degree. All these factors must then be added to determine the total uncertainty. The process is further refined by determining which factors add the most uncertainty and which add cost that may be unreasonable when attempting to perform a calibration. As mentioned above, under MIL-STD-45662A, it is usually 25% of the tolerance and is commonly called the 4:1 test accuracy ratio. While this is the goal, sometimes it is difficult to find anyone who can hold the 25%, and it may be necessary to accept lower ratios.

In addition, the accuracy of the calibration can be affected by the number of times the instrument under calibration is removed from the NIST-traceable master. For instance, if a company sends their light meter out to a company that had sent their meter out and their source is three or four times removed from NIST, the uncertainty may be larger than the allowable tolerance. The uncertainty for each time removed from NIST must be established. This is where many companies make the mistake of thinking that they are getting a valid NIST-traceable calibration, when in fact many calibration sources do not check the number of times removed or the uncertainty involved. This must be specified, since multiple sources may be involved. The use of analog meters or instruments to calibrate digital equipment should not be permitted. Analog equipment requires interpolation in between lines to attempt to obtain a reading. This is not accurate enough to calibrate digital equipment. The reverse, however, is permissible. The readout of digital equipment is three or four decimal places. This would allow for an accurate observation of units on an analog unit that cannot be read with digital accuracy.

WRITING PURCHASE ORDERS FOR CALIBRATION SERVICES

A written purchase order communicates all the requirements for calibration to the calibration source. Verbal purchase orders should be avoided entirely since they can leave too much open for interpretation and can cause problems later on if an instrument does not function within a specific range as expected. Written purchase orders should include the following items:

A blanket purchase order is an acceptable means of conveying this information. This way, the company does not have to develop a purchase order each time instruments are sent out. The blanket purchase order should have an expiration date, and also a review date (at least annually) for compliance. The company sending items out for calibration should provide a complete list of all nondestructive testing equipment or instruments to the calibration laboratory (ether internal or external), referencing the names, nomenclature, serial number or asset number, intervals of calibration, ranges of use, intended use, and the specification and/or manufacturer's tolerances. This will prevent noncompliances during subsequent audits.

VERIFYING STATUS OF EQUIPMENT AFTER CALIBRATION

After a piece of equipment is returned, the status of the calibration must be verified. This is basically a receiving inspection that is performed. All of the purchase order requirements listed above should be checked to make sure the calibration source recorded all the information requested. Experience gained by the authors over a number of years dictates that the test results should be reviewed, even though the calibration source has indicated acceptability. It is easy for a decimal point to be misplaced or information recorded that is out of tolerance and was missed during completion and final review of the certificate of calibration. This will prevent acceptance of data that is out of tolerance and that could possibly be discovered by an auditor later. If the instrument was actually sent back out of tolerance, then all hardware accepted during nondestructive tests using that instrument could be in question and could cause severe problems and expense, such as grounding aircraft, disassembly and retesting. Review of the dates of calibration of master equipment traceable to NIST is also important. It is not uncommon to find calibration sources that do not send their master equipment out on a frequent basis. For example, one calibration source listed the NIST number for their master equipment. When asked for the date of calibration, it was twenty years ago! So just having a NIST number is not enough.

There are also other methods for checking incoming calibrated equipment. If the instrument has a battery, turn the instrument on to see if the unit works. Sometimes placing the unit in a box or container for shipping after calibration can accidentally activate the "on" button. The unit would then be dead "as-received." Another easy check is to examine the box as received to see if any physical damage had occurred during shipping. If a delicate instrument was inside, the calibration could be affected. If a similar or like piece of equipment is available, compare the two side by side. For instance, if the ambient light in a penetrant testing booth is zero on one calibrated light meter and 32 lux (3 ftc) on another, there is a problem with one of the two meters, possibly the one just received from calibration. Another easy check for light meters is to turn on the unit and cover the sensor. It should read zero if all light is excluded.

A calibration review checklist encompassing the ten items listed under "Writing Purchase Orders," above, is a viable way to assure that the certificates of calibration are complete and provide the correct information. This checklist can then be attached to the certificate as documented evidence of review. See Table 1 for an example of a checklist.

Table 1 — Sample calibration certification review record.

OUT-OF-TOLERANCE CONDITIONS

When an instrument is received with an out-of-tolerance condition by the calibration source, a determination must be made concerning items that were accepted using equipment that was checked using the out-of-tolerance instrument. An impact assessment must be performed to determine if the out-of-tolerance condition is significant and adversely affects product quality, measurement integrity and/or safety. For example, let's say an ultrasonic light meter was sent out for calibration at +/- 5% of a standard, and it was reported that it was out of calibration by +1%. If the light reading is 20 µW/mm² (12.9 mW/in.²), 6% of that measurement would be 1.2 µW/mm² (774 µW/in.²). If a further reading was taken after a few days and it was 15 µW/mm² (9.7 mW/in.²), and then one subtracted 1.2 µW/mm² (774 µW/in.²) from that reading, the result would be 13.8 µW/mm² (8.9 mW/in.²). This is still above a 12 µW/mm² (7.7 mW/in.²) minimum reading some primary contractors require. That would mean that the out-of-tolerance condition did not have a significant effect on quality. This analysis would need to be written up and placed with the calibration data to demonstrate an acceptable condition despite the fact that the instrument was received in an out-of-tolerance condition.

CONCLUSION

While calibration may seem to be of minor importance, it can make or break the validity of a test or test system. Proper attention to the requirements for calibration can help reduce the possibility that this would contribute to poor or improper NDT. Of course, normal operations could also affect test equipment at any time during operation. Other factors such as age, misuse or amount of use could cause premature drift or out-of-tolerance conditions. Correct application of the specifications will facilitate better control of the calibration system and related equipment. Proper training of personnel to recognize problems and take preventative action (as opposed to having to take corrective action) is another part of the equation for a successful calibration system.

REFERENCES

ANSI Z540-1: General Requirements for Calibration Laboratories and Measuring and Test Equipment, Washington, DC, American National Standards Institute, 1994.

ISO 9000: Quality Management Systems — Fundamentals and Vocabulary, Geneva, International Organization for Standardization, 2005.

ISO 10012-1: Measurement Management Systems — Requirements for Measurement Processes and Measuring Equipment, Geneva, International Organization for Standardization, 2003.

ISO 17025, General Requirements for the Competence of Testing and Calibration Laboratories, Geneva, International Organization for Standardization, 2005.

ISO Guide 25: General Requirements for the Competence of Calibration and Testing Laboratories, Geneva, International Organization for Standardization, 1990.

MIL-C-45662A: Calibration Systems Requirements, Washington, DC, US Department of Defense, 1962.

MIL-HDBK-52A: Evaluation of Contractor's Calibration System, Washington, DC, US Department of Defense, 1984.

MIL-HDBK-52B: Evaluation of Contractor's Calibration System, Washington, DC, US Department of Defense, 1989.

MIL-STD-45662: Calibration Systems Requirements, Washington, DC, US Department of Defense, 1980.

MIL-STD-45662A: Calibration Systems Requirements, Washington, DC, US Department of Defense, 1995.

National Council of Standards Laboratories, "Papyrus Story," NCSL Web site, available at www.ncsli.org/misc/cubit.cfm, accessed 2006.

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* Alloyweld Inspection Company, Inc., 796 Maple Lane, Bensenville, IL 60106; (630) 595-2145; fax (630) 595-2128; e-mail skleven@alloyweldinspection.com.

^† Vastek Consulting, 7290 Jasmine Dr., Hanover Park, IL 60103; (630) 213-3432; fax (630) 213-3495; e-mail vastek@aol.com.

^‡ Packer Engineering, 1950 N. Washington St., Naperville, IL 60563; (630) 505-5722; fax (630) 505-1986; e-mail datkins@packereng.com.

 

Copyright © 2006 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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