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UKAS & ISO 17025 — what a real calibration certificate looks like
If a certificate doesn't carry a UKAS accreditation symbol and an accreditation number, it's not a UKAS calibration — regardless of what the supplier called it. Understanding the difference between 'calibrated', 'traceable' and 'UKAS-accredited' is essential when buying calibration services.
ISO/IEC 17025 in one paragraph
ISO/IEC 17025 is the international standard for the competence of testing and calibration laboratories. A lab that meets it has documented procedures, competent staff, controlled environment, traceable reference standards, validated methods and a measurement uncertainty calculated for every result. Most UKAS calibration labs are accredited to 17025.
What UKAS actually is
The United Kingdom Accreditation Service is the national accreditation body recognised by Government. UKAS assesses calibration laboratories against ISO/IEC 17025 and grants accreditation for a specific defined scope. A UKAS lab can only issue accredited certificates for the parameters, ranges and uncertainties listed in its schedule of accreditation.
Traceability to NPL
Every UKAS calibration is traceable through an unbroken chain of comparisons back to the National Physical Laboratory (NPL), which holds the UK's national measurement standards. Traceability without UKAS accreditation is possible but harder to demonstrate; the UKAS symbol is the shortcut to proving it.
Reading a certificate
A genuine UKAS certificate shows:
- UKAS accreditation symbol + accreditation number (e.g. 0123).
- The lab's accredited scope reference.
- Equipment identification — make, model, serial number.
- Date of calibration and recommended recalibration date.
- As-found readings (before adjustment) and as-left readings.
- Measurement uncertainty (typically expanded uncertainty at k=2, ~95% confidence).
- Reference standards used and their own traceability.
When you might not need UKAS
Not everything needs a UKAS certificate. Internal indicators, reference-only readings or low-stakes checks can be calibrated with a traceable (but not UKAS-accredited) certificate from a competent provider. If the measurement is for trade, customer evidence, accident investigation or critical compliance — pay for UKAS.