Calibration Certificate requirements...

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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.



• 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.

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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 %

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.


Types of calibration certificate.

When a 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 the quality 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’?


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.


In or Out of specification?

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).


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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.