“UKAS traceable” and “UKAS accredited” appear throughout leak testing specifications and quotations. They are not the same thing, and the difference occasionally causes real problems at audit.
This is a technical explainer on what traceability means for leak standards specifically. For the wider case on why calibration underpins compliance, see our companion article on why calibration is the heart of compliance.
What UKAS Is
The United Kingdom Accreditation Service is the UK’s national accreditation body, recognised by government. UKAS does not calibrate anything itself. It accredits laboratories, assessing them against ISO/IEC 17025 — the international standard for the competence of testing and calibration laboratories — and defines the specific scope each is accredited for.
Accredited vs Traceable — The Distinction That Matters
| Term | What it means |
|---|---|
| UKAS accredited calibration | Performed by a UKAS-accredited laboratory, within its accredited scope, issued on a certificate bearing the UKAS accreditation mark. The laboratory’s competence has been independently assessed. |
| Traceable calibration | Performed using equipment whose own calibration traces back through an unbroken chain to national standards. Competent and often entirely appropriate — but the laboratory itself is not accredited for that measurement. |
Both can be perfectly valid. The risk is a mismatch: if a customer drawing or quality procedure requires accredited calibration and you supply traceable, you have a non-conformance — regardless of how good the measurement was. Check which word your specification uses.
What the Traceability Chain Looks Like
For a leak standard, an unbroken chain typically runs:
- SI units — the internationally defined base units.
- National measurement standards — in the UK, maintained by NPL.
- Accredited calibration laboratory — reference standards calibrated against national standards.
- Your reference leak — calibrated against those reference standards, in line with ISO 20486.
- Your leak detector — verified against your reference leak.
- Your product — tested by that verified detector.
Every link needs valid documentation. One expired certificate anywhere breaks the chain, and everything downstream loses its evidential value.
Reading a Calibration Certificate Properly
A certificate is only as useful as the detail on it. Check for:
- Stated leak rate with units and uncertainty — a value without an uncertainty figure is incomplete.
- Reference temperature — leak rate is temperature-dependent; using the leak far from its stated temperature introduces error.
- Gas species — a rate for helium is not the rate for the same artefact with another gas.
- Calibration and recommended recalibration dates.
- Traceability statement — what the measurement traces back to.
- UKAS accreditation mark and schedule number, if accredited calibration is what you require.
Why Leak Standards Need Watching More Than Most Equipment
Leak standards are not stable indefinitely. Permeation-type leaks deplete as gas escapes by design — their rate changes measurably over time. Capillary and orifice leaks can partially block through contamination, which shifts the rate the other way. Both mean a leak standard genuinely needs periodic recalibration; it is not a “calibrate once” item.
Where Auditors Focus
Under ISO 9001, IATF 16949 (automotive) or ISO 13485 (medical devices), an auditor’s line of questioning is predictable. They will not simply ask whether the machine works — they will ask how you know it works. Expect: show me the calibration certificate for this detector; show me the certificate for the leak standard used to verify it; show me your verification records since that calibration; and show me what you do when a verification fails.
The last one catches people out. Having a documented reaction plan for a failed verification — quarantine, investigate, assess affected product — is as important as the certificates themselves.
Practical Recommendations
- Confirm whether your specification requires accredited or traceable calibration, and match it exactly.
- Keep a register of every leak standard with its rate, gas, temperature, certificate date and due date.
- Verify daily or per shift against a calibrated leak, and record the result.
- Harmonise standards across a mixed equipment fleet so a “leak” means the same thing on every machine.
- Document your response to failed verifications before you need it.
Related Reading
See calibration services for traceable calibration using certified leak standards, ISO 20486 explained for how reference leaks are calibrated, and pressure decay leak testing for verification in practice.
