A calibration certificate can be technically correct and still be rejected. The usual cause is one word in the tender: the specification asks for an accredited certificate, the supplier issues a traceable one, and the consultant finds out only when the authority sends the report back.
The two words are used as if they mean the same thing. They do not, and the difference is easy to check once you know where to look.
Traceable says where the reference came from. Accredited says an independent body has checked that the laboratory gets the right answer, for that instrument type, within a stated uncertainty.
The difference in one table
| Traceable calibration | Accredited calibration (ISO/IEC 17025) | |
|---|---|---|
| What it proves | The laboratory's references link to national or international standards through an unbroken chain | The same, plus an accreditation body has assessed the laboratory's competence, method and uncertainty for this specific calibration |
| Who checks the laboratory | Nobody independent; you rely on the laboratory's own statement | An accreditation body (in Thailand, TISI) that is a signatory to the ILAC Arrangement, with regular surveillance |
| Scope | Whatever the laboratory chooses to calibrate | Only the instrument types, parameters and ranges listed on the published scope of accreditation |
| Certificate | No accreditation mark | Accreditation mark and number; measurement uncertainty stated for every result |
| Accepted for | Most internal quality systems, routine field checks | Authorities, courts, tenders that specify ISO/IEC 17025, other accredited laboratories, and international recognition under ILAC |
What the rules actually require
ISO 9001:2015 (clause 7.1.5.2) requires measuring equipment to be calibrated or verified against standards traceable to international or national measurement standards. It does not require accreditation. An ISO 9001 auditor who accepts a traceable certificate is applying the standard correctly, and you are not obliged to pay for more.
ILAC-P10, the international policy on traceability, sets the order of preference for a laboratory that is itself accredited. Traceability should come from a national metrology institute or from an accredited calibration laboratory whose scope covers the service. Other routes are allowed only with evidence that the accreditation body has to assess. In practice this means that once a measurement report is issued by an accredited laboratory, or is going to court, the instruments behind it need accredited calibration.
Tenders and permits are where most rejections happen. Read the clause literally. "Calibrated by an ISO/IEC 17025 accredited laboratory" is not satisfied by a traceable certificate, even from a good laboratory. "Calibration traceable to national standards" is satisfied by either. If the clause says only "valid calibration certificate", ask the client which one they mean before the job starts, not after the report is rejected.
Check the scope, not the logo
Accreditation is not a property of the laboratory as a whole. It covers a list of calibrations. A laboratory can be accredited for sound level meters and still issue traceable-only certificates for accelerometers. Both certificates come from the same building with the same letterhead.
Acoustic Laboratory Thailand is a good example because its scope is narrow and published. Certificate 24-LB0054, issued by TISI and valid until 16 November 2028, covers three calibrations: sound level meters to IEC 61672-3, sound calibrators to IEC 60942, and measurement microphones to IEC 61094-2 and -6. Noise dosimeters, accelerometers, preamplifiers and data acquisition systems are calibrated traceably at ALT, and the certificate says so. That is how it should be. A laboratory that implies everything it does is accredited is the one to worry about.
Before you send an instrument
- Find the exact wording in the tender, permit condition or contract, and keep it with the order.
- Ask the laboratory for its scope of accreditation and confirm your instrument type and standard are on it. For a sound level meter that means IEC 61672-3 periodic testing, for a calibrator IEC 60942.
- Send the sound level meter together with its own calibrator, cables and microphone, so the system you use in the field is the system that was calibrated.
- Check the returned certificate for the accreditation mark, the stated uncertainty and the standard edition. A certificate without uncertainty is not an accredited result.
- Plan the interval. Annual is the common practice and what most authorities expect, but the interval is set by your own quality system, not by the laboratory.
A field check with a calibrator before and after each measurement is not a calibration of either. It confirms the system has not drifted on the day, and it is required by most measurement methods, but it does not replace the periodic laboratory calibration. ALT has a short note on the difference, linked below.
If you are not sure which certificate a job needs, send me the clause and the instrument model, and we will tell you before you ship anything. ALT's standard turnaround is 10 working days, with an express service when a deadline is close.
Questions we are asked
What is the difference between accredited and traceable calibration?
Traceable calibration means the laboratory's reference standards link to national or international standards through an unbroken chain. Accredited calibration (ISO/IEC 17025) adds an independent assessment of the laboratory's competence, method and measurement uncertainty for that specific calibration, by an accreditation body that is a signatory to the ILAC Arrangement. Only accredited certificates carry the accreditation mark and are accepted where a tender or authority specifies ISO/IEC 17025.
Does ISO 9001 require accredited calibration?
No. ISO 9001:2015 clause 7.1.5.2 requires measuring equipment to be calibrated or verified against standards traceable to international or national measurement standards. A traceable certificate satisfies ISO 9001; accreditation is required only when a customer, authority, tender or the measuring laboratory's own accreditation demands it.
Is every calibration from an accredited laboratory accredited?
No. Accreditation covers only the calibrations listed on the laboratory's published scope. The same laboratory can issue accredited certificates for some instrument types and traceable certificates for others. Check that the certificate carries the accreditation mark and states measurement uncertainty.
What does Acoustic Laboratory Thailand's accreditation cover?
ALT's ISO/IEC 17025 accreditation (certificate 24-LB0054, TISI, valid to 16 November 2028) covers calibration of sound level meters to IEC 61672-3, sound calibrators to IEC 60942 and measurement microphones to IEC 61094-2 and IEC 61094-6. Other instruments, such as noise dosimeters, accelerometers and data acquisition systems, are calibrated traceably and the certificate states this.
Is a field check with a calibrator the same as a calibration?
No. A field check before and after a measurement confirms that the measuring system has not drifted on the day. It does not replace periodic laboratory calibration of the sound level meter and of the calibrator itself.