Metrological Traceability Explained: What It Means and How to Check It

“Traceable to national standards” appears on almost every calibration certificate. It is also one of the most misused phrases in quality documentation.

The definition

The VIM defines metrological traceability as the property of a measurement result whereby it can be related to a reference through a documented, unbroken chain of calibrations, each contributing to the measurement uncertainty.

That definition has three parts, and all three must be present: | 1. | An unbroken chain. Your instrument was calibrated against a standard, that standard against a higher | one, and so on up to the realisation of the SI unit. | 2. | Documentation. Each link is supported by a calibration certificate. | 3. | Uncertainty at every step. Each calibration adds uncertainty. A certificate without a stated uncertainty | breaks the chain, whatever logo is printed on it.

What the chain looks like in practice

A typical chain for a factory pressure gauge:

SI unit (pascal) → national standards → accredited calibration laboratory’s reference standards → the factory’s master gauge → working gauges on the line

TEQ’s reference standards are traceable to the SI through recognized national or international standards.

Uncertainty grows at every step down the chain. That is why the choice of calibration laboratory matters: its best measurement capability (CMC) must be small enough, relative to your tolerance, to help with your own process.

How traceability is demonstrated

International practice, set out in ILAC P10, recognises traceability when calibration comes from:

• a national metrology institute whose capabilities are published in the BIPM key comparison database (KCDB) under the CIPM MRA, or
• a calibration laboratory accredited to ISO/IEC 17025 by an accreditation body that signs the international mutual recognition arrangement, now the Global ACI MRA, which replaced the ILAC MRA in

Other routes are possible, but the burden of proof then falls on the user.

Five checks on any certificate

1. Is the laboratory accredited, and is this calibration within its accredited scope? 2. Is the measurement uncertainty stated, with its coverage factor? 3. Are the reference standards used identified? 4. Is there a traceability statement naming the route to SI? 5. Do the calibrated points cover the range you use?

A certificate that says “traceable to NIST”, or to any other institute, but gives no uncertainty does not show traceability.