NIST traceable calibration: what it means and what to ask your calibration provider

What NIST traceable means

The idea comes from the International Vocabulary of Metrology (VIM, JCGM 200:2012). Entry 2.41 defines metrological traceability as the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. NIST adopts that definition in its own policy on metrological traceability.

NIST describes the phrase traceable to NIST as shorthand. It means something only when it stands for traceability through NIST to the SI or to another stated reference. In practice the chain runs from your instrument to the provider's reference standard, then through any laboratories in between, and on to standards maintained by NIST. Each step is a calibration with its own stated uncertainty.

Traceability belongs to a measurement result. NIST is explicit that an instrument, a standard, a laboratory or a company cannot itself be traceable. So a caliper is not NIST traceable. The readings on its certificate can be, when the chain and the uncertainty behind them are documented.

• Metrological traceability explained

What NIST traceable does not mean

• It does not mean NIST calibrated your instrument. NIST adds that even an instrument calibrated at NIST does not make your later results traceable by itself. The chain also needs stated uncertainties and a controlled transfer into your own measurements.
• It does not mean NIST approved the provider or the certificate. NIST states that it does not define, specify, assure or certify the traceability of measurement results other than those it provides itself.
• It does not mean the result is good enough for your tolerance. NIST and the VIM both note that traceability does not make the uncertainty small enough for a given purpose. A traceable result can still be too uncertain for the job.
• It does not prove anything on its own. Without the chain, the reference and the uncertainty behind it, the phrase is a label and not evidence.

Who has to prove it

NIST's policy splits the job in two. Supporting a traceability claim is the responsibility of whoever provides the result. For a calibration, that is the lab that issued the certificate. Assessing the claim is the responsibility of the user, which is you.

So a certificate that says NIST traceable starts a check rather than ending one. NIST's traceability FAQ lists what a claim should rest on: a clearly defined property that was measured, a full description of the measurement system, the result with its documented uncertainty, a complete specification of the reference, and measurement assurance for both the measurement system and the reference.

You do not have to audit each link yourself. Accreditation, covered below, is the usual way to get an independent check on the links you cannot see.

What a traceable calibration certificate should show

ISO/IEC 17025:2017 sets out what a calibration certificate from a lab working to the standard has to contain. Among the items clause 7.8.4.1 adds to the general report contents are the measurement uncertainty, the conditions that affect the results, a statement of how the measurements are metrologically traceable, and the results before and after any adjustment or repair. For a traceability check, look for the following.

• Results with units, each with its uncertainty and the coverage factor or confidence level.
• A traceability statement that names the route, for example to the SI through NIST or through another national metrology institute.
• The reference standards used, identified well enough that the lab can produce their own certificates when you ask.
• The calibration date, the method used and the environmental conditions where they affect the results.
• As-found and as-left results when the instrument was adjusted or repaired.
• For accredited work, the accreditation body's symbol or a text reference to the lab's accreditation. ILAC policy accepts that as evidence of metrological traceability when the calibration is inside the lab's scope of accreditation.
• The decision rule behind any pass or fail statement.

• Calibration certificate requirements and checklist

How ISO/IEC 17025 accreditation relates to the claim

Calibration labs are accredited to ISO/IEC 17025:2017. Clause 6.5 requires a lab to establish and maintain the metrological traceability of its results through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty, normally to the SI. Under ILAC policy the accreditation body assesses the lab's evidence for that traceability and for its uncertainty.

ILAC policy P10:07/2020 also says where the calibrations behind a lab's own equipment should come from when traceability is required. The two main routes are a national metrology institute whose service is covered by the CIPM MRA, or an accredited calibration lab whose scope covers that calibration and whose accreditation body is in the ILAC Arrangement. Other routes are for cases where neither is possible for a particular calibration.

Accreditation is strong evidence, but it is not a blanket endorsement. NIST says accreditation, including by its own NVLAP program, is a finding that a lab is competent. It does not mean NIST stands behind each traceability claim the lab makes. It also covers only the calibrations listed on the lab's scope.

A certificate from a lab that is not accredited can still be traceable. The evidence then has to come from the lab itself, and ILAC notes that you may need to assess that supplier much as an accreditation body would.

• ISO/IEC 17025 accreditation vs certification

Traceability outside the US

NIST is the national metrology institute of the United States. Other countries have their own, such as PTB in Germany, NPL in the United Kingdom, LNE in France, CEM in Spain, CENAM in Mexico, INMETRO in Brazil, IPQ in Portugal and NRC in Canada. The definition asks for traceability to a stated reference, normally the SI, and not to a particular institute. ILAC calls the idea that traceability belongs to a particular organization a persistent misconception.

National metrology institutes recognize each other's calibration and measurement certificates under the CIPM Mutual Recognition Arrangement (CIPM MRA), for the services listed in the BIPM key comparison database (KCDB). NIST notes that this recognition does not by itself make a result traceable to one institute traceable to another. If a contract names NIST, confirm that a chain through another institute is acceptable before you rely on one.

Accreditation follows the same pattern. Each country has a national accreditation body, for example DAkkS in Germany, UKAS in the United Kingdom, COFRAC in France, ENAC in Spain, ema in Mexico, Cgcre (part of INMETRO) in Brazil, IPAC in Portugal and SCC in Canada. Bodies that sign the ILAC Mutual Recognition Arrangement accept each other's accredited calibrations as equivalent, and ILAC publishes a search of current signatories.

Outside the US, a certificate will often state traceability to the SI through the national institute of the lab's own country. That is metrological traceability in exactly the same sense as a chain through NIST, and the same checks apply.

Questions to ask your calibration provider

Ask these before the first order, and put the answers on the purchase order or in a service agreement.

• Is this calibration on your scope of accreditation? Please send the scope document and name the accreditation body.
• Which reference standards will you use, and can you show me their current certificates?
• Where does your chain end, and through which national metrology institute does it reach the SI?
• What uncertainty will you report at my test points, and how does it compare with my tolerance?
• Will the certificate show as-found and as-left results, with an uncertainty for each point?
• If you state pass or fail, which decision rule do you use?
• Will any of the work be subcontracted, and will the certificate say so?
• How and when will you tell me if my instrument is found out of tolerance?

• Approving a calibration supplier
• Free approved supplier list template
• Check your TUR with the free calculator

Common mistakes

• Accepting a certificate number as proof. A number identifies a document. It does not show the chain, the reference standard or the uncertainty.
• Asking for a NIST test number. NIST says its test report numbers are for administrative use only. They should neither be used nor required as the sole proof of traceability.
• Taking the phrase as the evidence. A certificate that says NIST traceable but names no reference standard and states no uncertainty has not shown traceability.
• Relying on an expired chain. A reference standard used after its calibration due date is a link that is not current. NIST's weights and measures program expects the labs it recognizes to have a system that stops past-due standards from being used, and NIST says traceability needs to be reexamined periodically.
• Assuming every result is accredited. An accredited lab can also do work outside its scope of accreditation. Check that your parameter and range are on the scope.
• Treating traceable as good enough. A traceable result can still carry an uncertainty too large for your tolerance. Compare the two before you accept the instrument.

Where Axiospec fits

Axiospec is calibration management software. It records each calibration with its as-found and as-left readings, the reference standard used and the certificate, and it tracks when each instrument is due. It does not certify traceability or make a certificate accredited. Judging a traceability claim stays with you.