Accredited vs Non-Accredited Calibration: What Your Auditor Actually Checks

The difference between accredited and non-accredited calibration comes down to one thing: independent assurance. An accredited calibration — issued by a SAC-SINGLAS recognised laboratory under ISO/IEC 17025 — has had its methods, traceability and measurement uncertainty independently assessed by a national body. A non-accredited calibration produces a certificate, but with nothing independent vouching for the competence behind it. To an auditor, that distinction determines whether the certificate counts as evidence — or as paper.

What ISO/IEC 17025 accreditation actually means

ISO/IEC 17025 is the global standard for testing and calibration laboratories. When a laboratory achieves accreditation to this standard — in Singapore, granted by the Singapore Accreditation Council through the SINGLAS scheme — it means independent assessors have reviewed the lab's technical competence, its equipment and reference standards, its measurement methods, its uncertainty calculations and its quality management system. They return on a defined schedule to verify that the standard continues to be maintained. Accreditation is not a one-time certification; it is ongoing assurance.

The accreditation has a defined scope: each measurement parameter (pressure, temperature, electrical voltage, humidity, etc.) and measurement range is assessed separately. A laboratory can be accredited for electrical calibration but not for pressure, and a calibration certificate for pressure from that lab is not an accredited certificate — even if the lab's name is on the national accreditation register.

View Unitest's SAC-SINGLAS accreditation scope to confirm which parameters and ranges we are accredited for.

What your auditor actually checks on a calibration certificate

ISO 9001 and ISO/IEC 17025 auditors are trained to read calibration certificates. This is not a high-level review — they check specific items, and missing or incorrect information triggers a non-conformance finding. Here is exactly what they look for:

• Accreditation mark and number. Is a recognised accreditation mark (SAC-SINGLAS or equivalent MRA partner) present, with an accreditation number? Auditors cross-reference this against the national accreditation body's public register to confirm the lab is currently accredited and in good standing.
• Scope of accreditation for this parameter. Is the specific measurement parameter being calibrated — pressure, temperature, electrical, humidity — listed in the lab's accredited scope, within the stated range? "This lab is accredited" is not the same as "this calibration is accredited." The scope defines exactly what is covered.
• Traceability statement. The certificate must state that the measurement is traceable through an unbroken chain to national or international measurement standards. Without this, there is no basis for comparing your measurement to any other — and the certificate cannot support a claim of measurement accuracy.
• Measurement uncertainty. An accredited certificate must state the expanded measurement uncertainty for each calibration result, not just a pass or fail. Uncertainty tells you not just what the instrument read, but how confident you can be in that reading. A certificate without stated uncertainty is incomplete under ISO/IEC 17025 — full stop.
• As-found and as-left data. The certificate should show what the instrument was reading when it arrived (as-found) and after any adjustment (as-left). The as-found data is the item most often missing from non-accredited certificates — and it is the item most critical for impact assessment when an instrument turns out to have been out of tolerance.
• Clear instrument identification. Make, model, serial number, and preferably your own asset or equipment ID. Calibration date and the due date for the next calibration. Without unambiguous identification, the certificate cannot be definitively linked to a specific instrument in your records — which is exactly the kind of traceability gap that creates audit findings.

When non-accredited calibration is acceptable — and when it isn't

Not every measurement is safety- or quality-critical, and not every calibration needs to carry full ISO/IEC 17025 accreditation. For truly indicative measurements — where the reading is used for general awareness rather than to control a process, accept or reject a product, or satisfy a regulatory requirement — a non-accredited calibration may be acceptable within your quality system, provided your own risk assessment documents that decision explicitly.

Where it goes wrong is when companies use non-accredited calibration for critical instruments without a documented justification, then discover mid-audit that the certificates don't hold up. "We got a certificate" is not an adequate response when the auditor asks "is this measurement traceable and was the uncertainty assessed?" The fix — recalibrating retroactively with properly accredited certificates — is inconvenient, expensive, and may require a formal non-conformance investigation covering the period the instrument was in service.

The better approach is to assess each instrument against a simple question: if this instrument was wrong by its full tolerance, what is the worst-case consequence? If the answer involves product quality, safety, regulatory compliance, or contractual obligation — use accredited calibration. If the answer is "we'd probably notice and it wouldn't matter much" — a considered, documented decision to use non-accredited calibration may be defensible.

The most common mistakes companies make

After calibrating instruments for Singapore businesses across manufacturing, pharmaceutical, food and beverage, construction and engineering for many years, these are the errors we see most often:

• Assuming any certificate from any lab is an accredited certificate. Any provider can issue a document called a "calibration certificate." Only a SAC-SINGLAS accredited laboratory issuing work within its accredited scope produces an ISO/IEC 17025 accredited certificate. The header doesn't matter — the accreditation mark and scope reference do.
• Not checking whether the specific parameter is in the lab's scope. A lab may be accredited for electrical and temperature work, but not for pressure. Requesting pressure calibration from that lab produces a non-accredited result, even though the lab is generally "accredited."
• Accepting certificates without stated measurement uncertainty. Measurement uncertainty is not optional for accredited work. If the certificate gives only a pass/fail or only the measured value without an uncertainty, it is not compliant — regardless of what the accreditation mark says.
• Ignoring the as-found data. A certificate showing only the as-left reading tells you nothing about what the instrument was doing before it was adjusted. If the instrument was significantly out of tolerance, you need the as-found data to assess what happened during the period between calibrations.
• Letting certificates lapse. An instrument that was last calibrated two years ago on a 12-month interval has no valid calibration coverage for the second year. Any measurements taken in that period cannot be defended as having been made with a calibrated instrument.

How to verify a calibration laboratory in Singapore

The Singapore Accreditation Council maintains a public register of all SINGLAS-accredited laboratories, searchable by parameter and accreditation number. Before engaging a calibration provider for critical instruments, check three things: confirm their current accreditation status and expiry date, confirm that the parameter you need calibrated is listed in their current scope, and confirm the range in their scope covers the range of your instrument.

When requesting a quote, ask explicitly: "Is this calibration within your accredited scope? Can you provide your current scope of accreditation? Will the certificate state measurement uncertainty at each calibration point?" A reputable accredited laboratory will answer all three without hesitation.

Unitest Instruments is SAC-SINGLAS accredited under number LA-2023-0845-C. Our scope covers electrical, temperature, pressure and humidity calibration. Review our full accreditation scope and certificates before you request a quote — we want you to be confident before you commit.

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Don't wait for an audit finding to discover your certificates don't hold up. Request an accredited calibration quote or explore all our calibration services in Singapore.