ISO/IEC 17025 is the international standard that defines the competence of testing and calibration laboratories. When a lab is accredited to ISO/IEC 17025 (in Singapore, by SAC-SINGLAS), an independent national body has assessed and confirmed that the lab is technically competent, uses validated methods, maintains metrological traceability to national standards, and reports measurement uncertainty correctly. An accredited calibration certificate is therefore objective, third-party-verified evidence that your instrument reads true.

ISO/IEC 17025 vs a "traceable" or "in-house" calibration

Many suppliers issue a calibration certificate that is traceable but not accredited. The difference matters:

  • Accredited (ISO/IEC 17025): The lab's competence, methods, environment, staff and uncertainty budgets have been audited by an accreditation body. The certificate carries the accreditation mark and a schedule of accreditation.
  • Traceable only: The reference standards may trace back to a national institute, but no third party has verified the lab's actual measurement process or uncertainty.
  • In-house / manufacturer: Useful for routine checks, but typically not accepted as independent evidence in a regulated audit.

For quality systems (ISO 9001), GMP, aerospace, or any regulated environment, auditors generally expect ISO/IEC 17025-accredited calibration for measurements that affect product quality or safety.

What an accredited certificate must contain

A compliant ISO/IEC 17025 calibration certificate states the measured values, the reference standards used, the environmental conditions, the metrological traceability chain, and (critically), the measurement uncertainty for each point. It does not simply say "pass". It gives you the numbers so you can judge fitness for your own tolerance.

Why measurement uncertainty is the heart of it

Every measurement has uncertainty. ISO/IEC 17025 requires the lab to calculate and report it, so you can apply a proper guard band when deciding whether your instrument is still fit for purpose. A certificate without uncertainty is incomplete. You cannot make a defensible pass/fail decision from it.

Accreditation in Singapore: SAC-SINGLAS

In Singapore, laboratories are accredited under the Singapore Accreditation Council (SAC) SINGLAS scheme. SAC is a signatory to the ILAC Mutual Recognition Arrangement, which means a SAC-SINGLAS calibration certificate is recognised internationally. Important for exporters and multinational quality systems.

What the accreditation audit actually checks

Accreditation is not a one-time paperwork exercise. SAC assessors carry out an initial on-site assessment against the ISO/IEC 17025 clauses, then return for periodic surveillance visits (typically annually) and a full reassessment on a multi-year cycle, usually every four years. What they examine goes well beyond the calibration bench: the lab's management system and document control, staff competence records and training files, environmental monitoring (temperature and humidity logs for the calibration room itself), the maintenance and inter-comparison history of every reference standard, the lab's measurement uncertainty budget for each calibration it offers, and its participation in proficiency testing or inter-laboratory comparisons that independently verify its results against other accredited labs. A lab that fails a surveillance visit can have specific scopes suspended or withdrawn until it corrects the finding, which is exactly why an accredited certificate carries weight that a "calibrated in-house" sticker does not.

Reading the schedule of accreditation: scope matters

Accreditation is granted for a specific scope, not a blanket approval to calibrate anything. SAC publishes each lab's schedule of accreditation listing the exact parameters, ranges and uncertainties it is accredited for, for example, DC voltage from 0 to 1000 V at a stated uncertainty, or temperature from -20°C to 250°C using a specific reference method. A calibration certificate for a measurement outside that published scope, even if it carries the accreditation logo, is not an accredited result for that parameter. Before accepting a certificate as audit evidence, check the lab's current scope (published on the SAC website) against the actual measurement you needed calibrated. This catches a surprising number of certificates that look accredited but cover the wrong range or the wrong quantity for the instrument in question.

Worked example: why the uncertainty number changes the decision

Consider a digital multimeter specified by its manufacturer to ±0.5% accuracy on the 100 V DC range, used in a process where the acceptable tolerance is ±1%. An accredited calibration returns a reading of 100.3 V against a nominal 100 V input, with a stated expanded measurement uncertainty of ±0.08 V. The instrument is comfortably within tolerance, and because the uncertainty is small relative to the tolerance band, there's no ambiguity in the pass decision. Now imagine a second instrument, also reading 100.3 V, but from a lab that does not report uncertainty and simply states "Pass." You have no way to know whether that instrument is genuinely within tolerance with margin, or borderline and one small drift away from a fail; you're trusting the lab's own undocumented judgement instead of making your own defensible decision from the numbers. This is the practical, day-to-day reason regulated industries insist on accredited certificates with stated uncertainty rather than a bare pass/fail stamp.

When you need ISO/IEC 17025 calibration

  • Instruments used to release product, or to make safety or compliance decisions
  • Measurements cited in an ISO 9001, GMP, HACCP or regulatory audit
  • Reference instruments used to check or calibrate other equipment
  • Any situation where a customer, regulator or auditor asks for "accredited" calibration

Unitest Instruments operates a SAC-SINGLAS accredited calibration laboratory (ISO/IEC 17025). If you are unsure whether your application needs accredited calibration, send us the instrument list and the standard you are being audited against and we will advise. You can also read our guide on how to read a calibration certificate for a line-by-line breakdown of what a compliant certificate should contain, or browse our calibration services directly.