Calibration certificates

What a calibration certificate establishes

A calibration certificate reports an identified instrument's response under stated conditions, with uncertainty. VIM defines calibration as establishing a relationship between reference values and indications, then obtaining measurement results from that relationship. Corrections, tables, curves and functions can express it. BIPM/JCGM, VIM 2.39

Calibration characterizes response; adjustment changes it; verification assesses fulfillment of specified requirements. A calibrated instrument can fail an accuracy requirement, and calibration does not imply adjustment. BIPM/JCGM, VIM 2.39, annotations dated 4 September 2015

Worked example: correction versus acceptance

In this example, a +0.008 mm indication error against a ±0.010 mm limit passes simple acceptance. A rule requiring the entire interval to fit rejects it when expanded uncertainty is 0.004 mm, because the upper bound reaches +0.012 mm. At a reference of 10.000 mm, the instrument indicates 10.008 mm:

• Indication error: 10.008 − 10.000 = +0.008 mm.
• Additive correction: −0.008 mm.
• Reported expanded uncertainty: assumed U = 0.004 mm, with stated k = 2, so uc = 0.002 mm.

For a later indication of 10.006 mm, the corrected estimate would be 9.998 mm, if the same correction remains applicable locally under equivalent conditions. This calculation does not justify extrapolating that correction to 100 mm, nor does applying it eliminate uncertainty. It also does not establish the complete uncertainty of the later measurement.

With the interval rule, the upper bound on an acceptable measured error is +0.006 mm and the lower bound is −0.006 mm. The limits leave 0.004 mm of room for expanded uncertainty at either end.

The interval rule rejects the result because of the agreed acceptance boundary. The true error remains uncertain. ILAC G8 relates conformity decisions to the named requirement, the decision rule and the risks of false acceptance and rejection. ILAC G8:09/2019, §§1, 2 and 4

Results have a defined scope

A numerical result belongs to a particular measurement: quantity, range or test point, instrument configuration and conditions. A certificate's identity and method information connect its numbers to that measurement. A correction established near one point does not, by itself, establish the correction throughout an instrument's range. This is an explanatory consequence of calibration being a relationship established under specified conditions, rather than a blanket declaration of accuracy. BIPM/JCGM, VIM 2.39

“As submitted” and subsequent results describe different states when an intervention changes instrument response. A post-adjustment result cannot alone establish the error that existed before adjustment. NIST's interval guidance explicitly discusses both as-submitted and post-calibration accuracy and precision data as evidence of time-dependent performance. A calibration date consequently identifies an event; it is not a prediction that the same response will persist indefinitely. NIST, Recommended Calibration Interval

Uncertainty and accreditation scope

ISO/IEC 17025 concerns the competence, impartiality and consistent operation of testing and calibration laboratories. Its scope covers laboratory activities, rather than certifying every product measured with a laboratory-calibrated instrument. The current ISO record identifies the third edition, November 2017, confirmed in 2023. Its 2018 corrections concern the French and Spanish versions; ISO explicitly says the English version is unaffected. The public abstract establishes that scope; it does not reproduce the standard's complete reporting requirements. ISO/IEC 17025:2017

Measurement uncertainty describes the dispersion associated with a result; it is not the same number as an observed indication error or a permissible tolerance. ILAC P14 specifies reporting the measured value with expanded uncertainty, units, coverage factor and coverage probability. Expanded uncertainty is commonly written U = k × uc, where uc is combined standard uncertainty. The coverage statement matters: a factor of two does not, without the relevant distribution and evaluation, establish an exact universal probability. ILAC P14:09/2020, §5.2; ILAC G8:09/2019, §1.12

An accredited laboratory's calibration and measurement capability (CMC) describes its scope through the measurand, method, range or other parameters, and uncertainty. It is not automatically the uncertainty delivered for every customer's instrument. P14 includes contributions attributable to the actual device and relevant short-term calibration effects; reported uncertainty tends to exceed the CMC. Unknown transport effects are normally excluded. The uncertainty of a later application can therefore differ from the certificate's calibration uncertainty. ILAC P14:09/2020, §§4.1–4.3 and 5.4–5.6

Traceability does not establish fitness for every use

Metrological traceability is a property of a measurement result, connected to a reference through a documented, unbroken chain of calibrations, each contributing uncertainty. It is distinct from tracing an instrument's ownership or a document's history. NIST explains that a NIST-calibrated instrument does not automatically make every subsequent measurement traceable. NIST also does not certify other organizations' traceability claims merely because they refer to its services. NIST, Metrological Traceability: Frequently Asked Questions and Policy

Traceability and fitness for purpose answer different questions. A traceable result can have uncertainty too large for the application, and traceability does not establish freedom from mistakes. Thus, an instrument suitable for sorting broad size categories may provide insufficient evidence for a much narrower dimensional tolerance, despite having the same certificate. The latter comparison is explanatory synthesis; the underlying distinction is explicit in NIST's policy and FAQ. NIST, §§3 and 5.1.4

Time and continuing applicability

There is no universal NIST annual recalibration requirement. NIST identifies accuracy needs, contracts or regulations, inherent instrument stability and environment as factors affecting intervals. A scheduled due date expresses an applicable calibration program or requirement; it does not turn the preceding certificate into a guarantee of unchanged performance until that date. Conversely, a historical result remains evidence of the measured state even after a scheduled interval has elapsed. NIST, Recommended Calibration Interval, updated 29 May 2026

Source versions

The ILAC PDFs identify G8:09/2019 and P14:09/2020 on their covers. G8 supplies conformity-decision guidance; P14 is ILAC policy for calibration uncertainty within its accreditation framework. The VIM page distinguishes its definition from informative annotations dated 4 September 2015. These sources support the distinctions above without making a calibration certificate a universal product approval or acceptance decision.