Calibration Certificate requirements...

Introduction.

The calibration certificate contains crucial evidence of the integrity of test equipment and the validity of a calibration. This is despite the fact that the need to calibrate equipment is considered by some to be simply a ‘necessary evil’ to attain quality. Companies in a variety of worldwide industries spend hundreds of millions of dollars annually to calibrate their instrumentation yet the tangible evidence of what has been obtained in return for this investment generally receives scant attention. Except for a cursory glance before consignment to the filing cabinet, where it lies awaiting retrieval in the unlikely event of scrutiny during an audit, the calibration record is rarely reviewed after its receipt. One reason is the mistaken assumption that all calibrations are alike, but another is a lack of knowledge about what to look for and its significance.

 

Why is Calibration necessary?

As components age and equipment undergoes changes in temperature or sustains mechanical stress, critical performance gradually degrades. This is called drift and, when it happens, test results obtained using measurement equipment become unreliable and both design and production quality suffers. Whilst drift cannot be eliminated, it can be detected and contained through the process of calibration.

 

Calibration is defined as a performance comparison against a standard of known accuracy.

It may just involve determination of the deviation from nominal or include correction (adjustment) to minimize the observed error. Properly calibrated equipment promotes confidence that manufactured products and support services meet their specifications.

 
Calibration Certification. Change Control. Standards. Documentation….. Information and training presentation.
 

 

Calibration:

• Increases production yields,
• Optimizes resources,
• Assures consistency,
• Is fundamental to compliance with international, regulatory or industry-sector specific standards that require measurements to be ‘traceable to national standards’ and, in doing so,
• Ensures measurements (and perhaps products) are compatible with those made elsewhere.

 

Traceability:

A hierarchical relationship is achieved by calibrating equipment through the use of standards of better accuracy.
It could be illustrated like this:
National Standard……………………… Accurate to 0.02 %
Calibration Laboratory………………………………….0.13 %
Production Equipment…..…………………………..0.6 %
Manufactured Product……………………………….…8 %

Sample Calibration Procedure >>>
Calibration Process Requirements >>>
 

Of course, these calibrations need to be done on a planned, periodic basis with evidence of the comparison results maintained. This record must include identification of the specific standards used (which must be within their assigned calibration interval) and some means of knowing the method used and other test conditions. By examining these records, it should be possible to demonstrate an unbroken chain of comparisons that ends at the agency responsible for maintaining and developing a country’s measurement standards (now generically known as a national metrology institute). This demonstrable linkage to national standards, with known accuracy, represents ‘traceability’. In fact, it doesn’t stop there because these laboratories routinely undertake international comparisons which help to establish worldwide consensus on the accepted value of the fundamental measurement units. The result of it all, is a coherent measurement system that ensures the compatibility of components produced in multiple locations around the world.

 

Types of calibration certificate.

When a manufacturing calibration is performed, the certificate or report is the end product and represents the only tangible evidence of the service that the purchaser can link to the expenditure. Its importance is obvious for that reason alone. Not only does it show the results obtained across the scope of testing, it is also a key means of judging thequality of the calibration provider’s service.

Most calibration laboratories offer several forms of certificate but the apparent similarity in their names can be confusing. Before placing an order, be sure to understand exactly what’s on offer to ensure it meets your need.
For example, what’s the difference between a ‘Certificate of Calibration’ (or maybe a ‘Measurement Report’), or certificates of ‘Verification’ or ‘Conformance’?
Similar to the often-confused metrological terms accuracy and precision, there is a myth that calibration and verification are differentiated on the basis of quality or integrity. The perception being that ‘verification’ is a quick-check of performance, perhaps made without any real traceability, whereas ‘calibration’ provides genuine assurance that the product really met specification. In fact, the US national standard ANSI/NCSL-Z540 defines “verification” as being calibration and evaluation of conformity against a specification. This definition originated with the now obsolete ISO/IEC Guide 25 but neither its replacement, ISO/IEC17025:1999 [4] or the International Vocabulary of Measurement [1] currently have it or any alternative. The only relevant international standard that includes terminology covering the process of both calibrating and evaluating a measuring instrument’s performance against established criteria is ISO 10012 which uses the rather cumbersome term “metrological confirmation”.

As already mentioned, calibration is simply the process of comparing with a known standard and reporting the results.

For example:
Applied = 1.30V, Indicated = 1.26V (or Error = -0.04V)

Calibration may include adjustment to correct any deviation from the value of the standard.
Verification, as it relates to calibration, is evaluation of these results against a specification, usually the manufacturer’s published performance figures for the product.

For example:
Error = -0.04V, Spec. = ±0.03V, ‘FAIL’

Some laboratories include a specification compliance status statement on the Certificate of Calibration. (i.e. the item did or did not comply with a particular specification).
Where no judgment is made about compliance, or correction has not been made to minimize error, it has been suggested that ‘Certificate of Measurement’ would be a more descriptive title to aid recognition of the service actually performed. Some suppliers also use ‘Certificate of Verification’ where no measurements are involved in
the performance testing, rather than ‘Certificate of Functional Test’ as this latter term is often considered to be limited.
So, what is a ‘Certificate of Conformance’? Typically available when an instrument is purchased, it is now generally recognized that such a document has little value as an assurance of product performance. Of course, the manufacturer expects that the product conforms to its specification but, in this sense, the document simply affirms that the stipulations of the customer’s purchase order or contract has been duly fulfilled.

 
Calibration Certification. Change Control. Standards. Documentation….. Information and training presentation.
 

FMEA. Failure Mode Effects Analysis >>>
Statistical Process Control. Information and training presentation >>>
Quality Risk Management >>> 
 

Extent of Testing.

Standards have existed for many years that define characteristics and qualification of certain measurement devices, such as ISO 3650 for metric gage blocks. In contrast, there is no such formality concerning what constitutes ‘calibration’ for the type of instrumentation used in the electronics industry, like spectrum analyzers and multimeters. Calibration requests often use a phrase like “calibrate to manufacturer’s specification” but this can be variably interpreted. If taken literally, it’s possible that the user would never see the item again as exhaustive testing against all of the published specifications could take a very long time and include probably irrelevant detail such as its weight, as well as being tested to destruction if shock-tests are involved! Clearly, there must be a compromise between gaining adequate confidence in the product’s performance and under- or over-testing. If the manufacturer publishes performance test recommendations, this can be a good guide for both the equipment user and the calibration laboratory but it shouldn’t be taken for granted that these are followed.

Remember that the onus of responsibility that the calibration is adequate for the use made of the equipment rests with the user, not the supplier.

 

In or Out of specification?

Despite what some suppliers may claim, no calibration is perfect! There is always some degree of uncertainty about the ‘true’ value of a measurement. Contributors to this ‘potential for inaccuracy’ include the performance of the equipment used to make the measurement, the test process itself and environmental effects.
Additional imprecision may result from behaviour of the item being measured. A skilled metrologist will assess and combine these various contributions in an uncertainty budget. To prove that a product complies with specification (or does not), the uncertainty must be less than the specification of the unit under test (UUT).

There are several practices regarding how uncertainty is treated when deciding whether a UUT meets specification. One of the traditional practices has been, in effect, to ignore uncertainty if it doesn’t exceed one quarter of the UUT specification. The justification for this approach originates from consumer and producer risk analysis about which David Deavor has written several respected papers. Where this 4:1 test uncertainty ratio rule can’t be met, he explains how a guardband limit may be calculated to maintain consumer risk at the 4:1 level or as desired.
A shared-risk approach is not, however, acceptable to some equipment-users or accreditation bodies. The ILAC-G8 requirement that has been adopted by European accreditation agencies, for example, is for the acceptance limit (for pass/fail determination) to be the difference between the specification and uncertainty.

 

A Standard for Calibration Certificate requirements.

Although several accreditation bodies had prescribed the format and content for calibration certificates (and continue to do so) it wasn’t until the publication of ISO/IEC17025 that there was worldwide consensus in the form of a standard. However, it wouldn’t be fair to say that all calibration certificates are identical, even if they comply with the standard. In fact, a deliberate amount of flexibility is allowed by the criteria depending upon customer requirements and commercial service-level options. Provided that there is customer agreement,

these optional attributes are:

• The certificate may be of a “simplified” form where only limited detail is included (although the records must be complete).
• If a statement of compliance with a stated specification is made, it isn’t necessary to report measured values or the uncertainty.
• The method of “taking uncertainty into account” in respect of compliance statements is not prescribed.
• Recommended re-calibration dates may be quoted on the certificate or label.
• Hardcopy or electronic format is acceptable.
Also, it’s worth noting that the standard doesn’t define all elements that might otherwise have been assumed to represent a calibration service.
• Adjustment to return a product to a state of specification compliance is not necessary.
• Inclusion of a compliance statement is not necessary; the measured values and uncertainties only are reported.
• A calibration label or integrity seals are not mandatory.
• The extent of testing or adequacy for particular applications.

 

Value and limitation of accredited calibration.

An independent, expert body may assess the technical competence and quality system of a calibration provider. The international standard, ISO/IEC 17025, provides the principal criteria and some agencies have additional “amplification” documents that establish certain expectations where, it is considered, the standard lacks detail. A good example of this is UKAS-LAB5 covering certificate and label style. Successful candidates are said to have gained the agency’s accreditation for a specific measurement capability (i.e. parameter, range, method and uncertainty). The United Kingdom Accreditation Service has referred to the provision of accredited calibration as “authenticated traceability”.

A worldwide agreement between such accreditation bodies, the so-called ‘ILAC Arrangement’, means that the validity of manufacturing calibration certificates issued under the authority of any member is recognized in any other signatory’s country. Inclusion of the accreditation body’s logo on a calibration certificate aids its acceptance.
Initially it seems as though a certificate bearing the brand-mark of a bona fide accreditation body should be an ultimate solution for users wanting confidence in the performance of their equipment. Ideally, of course, it would be but seldom in life are things straightforward and accredited calibration is no exception. One potential danger for the unaware seeker of calibration services is that the extent of testing may be limited by the scope of accreditation. This could be in terms of tested functions or ranges or because the measurement uncertainty is larger than the specification. Furthermore, some accreditors dictate the proportion of measurements that may be included in the calibration certificate that are beyond the laboratory’s recognized scope, even if they are clearly annotated as such. The effect of this may be that separate certificates are provided for accredited and non-accredited tests which is cumbersome to deliver and confusing to the customer or, perhaps more likely, only limited testing is performed in comparison to the product’s potential capability. Accreditation bodies cannot stipulate what constitutes appropriate calibration for the majority of users so informed negotiation with the laboratory is necessary. The consequence of all this could be reduced confidence in the equipment’s overall performance. Much has been written about the challenges that accompany provision of accredited calibration outside the confines of the traditional standards laboratory and when dealing with complex, multifunctional equipment found in the traceability paths of modern manufactured products.

 
Calibration Certification. Change Control. Standards. Documentation….. Information and training presentation.
 

 

The key elements to look for on a calibration certificate.

The supplier’s contact details – Essential information in case of a query about the calibration.
Service type – Most suppliers offer a variety of service levels with different
attributes.
Unique tracking and/or certificate number – The supplier’s key to all their records. A crucial aspect of traceability.
Equipment identification – Calibrations relate to an individual item of equipment, not a product sample.
Customer identification – If the equipment was subcontracted, your supplier will be stated as the customer and there may also be indication that it was on your
behalf.
Test location – Identifies whether it was performed at the customer’s premises
(“on-site” and perhaps in-situ which may limit the scope of testing), the supplier’s particular branch or depot, or whether a subcontractor was used. Measurements made by a subcontractor may or may not be transparent to the end-customer so it’s wise to seek assurances about your supplier’s policy.

Test date – Some calibrations take more than 1 day so will generally be the
calibration end-date. The date from which users determine the next
due calibration. A calibration due date may also be included but may only be for
general guidance based on the manufacturer’s original suggested calibration interval rather than specifically for this item.

Test conditions – Temperature, humidity and sometimes altitude (pressure) are most
often found as they can affect equipment characteristics. A single value without a control limit is meaningless. Parenthesis are sometimes included to indicate that the control limit is not a measurement uncertainty but this format is by no means common.

Test methodology – Reference to a documented procedure is an aspect of traceability.
Like the record of calibration standards used, it enables precise repetition of the test in case of query.

Standards conformity – Affirmation of compliance with, or independent certification against, international conformity assessment standards such as ISO9001 or
ISO17025. This may, of course, depend upon the chosen service level.

Performance summary – A statement that clearly indicates whether or not the equipment met stated criteria, upon receipt and at completion of calibration (i.e.
after any repair/adjustment). May include concise details of tests that failed to meet defined criteria. May depend upon the chosen service level.

Scope of testing – Indication of the tested functions and their status, maybe in
conjunction with a measurement report. It should particularly identify significant omissions from the manufacturer’s recommendations (if any), standards or known user requirements.

Confidence level – Concerns the likelihood that a compliance decision is correct and
relates to the measurement uncertainty and its adequacy in relation to the specification. Through international consensus this is typically 95% (a so-called ‘coverage factor’ of 2) but may need to be higher in usage applications with high sensitivity to risk (e.g. safety critical). Applications that are more tolerant may accept a lower probability.

Acceptance limits used or reference publication – Often citing a published specification, it enables confirmation that the appropriate criteria was used for the customer’s particular application.

Print or issue date – If amendments are later found necessary, a later issue date should
appear to maintain a chronological paper trail.

Page numbering – Enables confirmation that a complete record has been received

Details concerning traceability to international measurement standards – Usually an affirmation concerning the national metrology institutes to which, ultimately, the supplier’s calibration standards can be related through an “unbroken chain of comparisons of stated accuracy”.

Measured values, particularly for nonconforming tests – Necessary if correctional data is to be used and also for test points that fail to meet specification. The degree of deviation is needed to establish whether re-work or product recall is necessary if integrity has been compromised. May be service level dependent.

Clarity of acceptance (or specification) limits and measured values – Is it easy to verify the equipment’s status? It is if measured values and test limits are in the same format and units. The measurement resolution must be appropriate to that of the test limit.

 

Conclusion.

The aim of this article was to improve the perceived value of the tangible evidence resulting from manufacturing calibration – the certificate or measurement report – by identifying the various components that may appear and explaining their significance to the equipment user. By doing this, it is hoped that a greater appreciation of good certificates will be developed and confidence gained to knowledgeably challenge a supplier when something else is provided.

Sample Calibration Procedure >>>
Calibration Process Requirements >>>
 

Reference sources:
Reference: Agilent Technologies UK Ltd., Winnersh, Berks. RG41 5DZ

International Organization for Standardization; International Vocabulary of Basic and General Terms in Metrology.

Deavor, David; Maintaining Your Confidence & Guardbanding with Confidence & Managing Calibration Confidence in the Real World. NCSL Workshop & Symposia 1993/4/5.

International Laboratory Accreditation Cooperation; Guidelines on Assessment and Reporting of Compliance with Specification.

International Organization for Standardization & International Electrotechnical Commission; General Requirements for the Competence of Testing and Calibration Laboratories.

United Kingdom Accreditation Service; Reporting of Calibration Results.
Hutchins, Mike; Challenges of an Accreditation System in a High-volume, Production-oriented Environment. NCSL Workshop & Symposium 1995.

Abell, Dave; Meeting 17025 Requirements for Complex Electronic Test Equipment. NCSL International Workshop & Symposium 2002.

International Organization for Standardization; Guide to the Expression of Uncertainty in Measurement.