Calibration establishes the relationship between what an instrument indicates and the values of traceable measurement standards, with a stated measurement uncertainty. Verification provides objective evidence that an item meets specified requirements, usually a pass or fail against a tolerance.
Put simply, calibration tells you how far out an instrument is and how confident you can be in that figure. Verification tells you whether that performance is acceptable for your work.
Mixing the two up is a frequent source of findings under clause 6.4 of ISO/IEC 17025:2017. Worse, it can mean results are reported from equipment that was never shown to be fit for purpose.
The definitions in plain terms
The terms come from the International Vocabulary of Metrology (VIM, JCGM 200:2012), which ISO/IEC 17025 relies on.
- Calibration: comparing an instrument's indications with measurement standards of known uncertainty, then using that relationship to obtain correct results. The output is errors or corrections plus uncertainty, not a pass or fail.
- Verification: objective evidence that an item fulfils specified requirements, such as a balance reading within ±0.5 mg of a certified 100 g weight.
- Adjustment: operations that change an instrument so it indicates prescribed values, such as a balance's internal adjustment or a technician resetting a thermometer offset. The VIM is clear that adjustment is not calibration.
- Intermediate check: a check between calibrations to maintain confidence that equipment is still performing, required by ISO/IEC 17025:2017 clause 6.4.10 when necessary.
A useful way to connect them: calibrate, review the certificate against your requirements (that review is a verification), adjust if needed, then use intermediate checks to show nothing has drifted before the next calibration.
What ISO/IEC 17025 and ISO 15189 require
In ISO/IEC 17025:2017, clause 6.4 covers equipment and clause 6.5 covers metrological traceability. The points assessors test most often are:
- 6.4.6: equipment must be calibrated when its accuracy or measurement uncertainty affects the validity of results, or when calibration is needed to establish traceability.
- 6.4.7: a calibration programme, reviewed and adjusted as necessary.
- 6.4.8: calibration status labelled or otherwise identified.
- 6.4.10: intermediate checks carried out according to a procedure.
- 6.4.11: correction factors from calibration updated and applied.
- 6.5: traceability to the SI through calibration by a competent laboratory, certified values of reference materials from a competent producer, or direct realisation of SI units.
ISO 15189:2022 mirrors this for medical laboratories in clause 6.4 (equipment) and clause 6.5 (equipment calibration and metrological traceability). In practice, analyser calibration usually relies on manufacturer calibrators with documented traceability, and the lab must show it has reviewed that evidence.
Calibration vs verification: comparison table
| Aspect | Calibration | Verification |
|---|---|---|
| Purpose | Establish the relationship between indications and reference values | Confirm the item meets specified requirements |
| Output | Errors or corrections with measurement uncertainty | Pass or fail against a stated tolerance |
| Who does it | Usually an accredited calibration laboratory, or a competent in-house function | Often trained lab staff, in-house |
| Traceability | Must be demonstrated (clause 6.5) | Depends on calibrated reference items used in the check |
| Uncertainty stated? | Yes | Not normally, but it should inform the tolerance and decision rule |
| Frequency | Defined interval, based on use, risk and history | Before use, daily, weekly, or after repair or relocation |
| Example | Balance calibrated across its range | Daily check with a certified 100 g weight |
| Record | Certificate, plus documented review | Check log with result, tolerance, date and initials |
Where method validation and verification fit
Equipment verification is not the same as method verification. Method verification shows that a standard method performs as expected in your lab, while validation establishes performance for a non-standard or modified method. Calibrated, verified equipment is a prerequisite for both. See method validation vs verification explained for the detail.
Practical examples by equipment
| Equipment | Calibration | Verification or intermediate check |
|---|---|---|
| Analytical balance | Accredited calibration across the working range, with uncertainty | Daily or before-use check with certified weights; periodic repeatability and eccentricity checks |
| Pipettes | Gravimetric calibration to ISO 8655 by a competent provider | In-house gravimetric checks at set volumes between calibrations |
| Thermometers and data loggers | Calibration of a reference thermometer at points relevant to use | Comparison of working thermometers against the reference |
| Incubators | Not calibrated as a whole; the probe or logger used is calibrated | Temperature checks and periodic mapping against the required range, such as 37 ± 1 °C |
| pH meter | Periodic electronic and probe performance check by a service provider, where needed | Daily buffer adjustment followed by a check buffer read against tolerance |
The pH meter causes the most confusion. The daily routine with two or three buffers is usually called calibration, but in VIM terms it is adjustment. Reading a separate check buffer afterwards is the verification, and the buffers themselves should be traceable reference materials.
Worked example: a drifting balance in a Dubai food testing lab
A food testing laboratory in Dubai weighs samples for gravimetric fat and moisture on an analytical balance with 0.1 mg readability. The daily intermediate check uses a certified 100 g weight with a tolerance of ±0.5 mg, set from the method's requirements and the balance's calibration uncertainty.
Over two weeks the check results creep upwards: +0.2, +0.3, +0.4 mg, then +0.6 mg. The analyst stops, labels the balance out of service and informs the quality manager, as clause 6.4.9 requires for equipment giving questionable results.
The investigation finds an air-conditioning vent recently redirected towards the bench. The balance is relocated, levelled and cleaned, and the internal adjustment is run. The check still reads slightly high, so the accredited calibration provider is called. Their as-found data confirms the error, and the as-left results after adjustment are within tolerance.
The lab then assesses impact under clause 7.10, nonconforming work. It reviews results produced since the last check that showed a clear margin, works out the effect of a 0.6 mg error on typical sample and extract masses, and decides whether any reports need review. Check frequency is increased for a month, and the trend chart becomes part of routine review.
Common audit findings
- Calling the pH meter's buffer routine calibration, with no check buffer and no traceable buffers.
- Certificates filed without review, even when the reported error exceeds the lab's own tolerance.
- Calibration providers used without evidence of competence or traceability.
- No as-found data, so the impact of drift cannot be assessed.
- Intermediate checks performed with uncalibrated weights or thermometers.
- Tolerances never defined, so checks are simply ticked as acceptable.
- Correction factors on certificates not applied, contrary to clause 6.4.11.
- Calibration intervals copied from the manufacturer and never reviewed against history.
Key takeaway: Calibration gives you traceable numbers with uncertainty; verification decides whether those numbers are good enough. You need both, plus intermediate checks, to defend every result.
Frequently Asked Questions
What is the main difference between calibration and verification?
Calibration compares an instrument with traceable measurement standards and reports errors or corrections with measurement uncertainty. Verification compares performance against specified requirements and gives a pass or fail. A calibration certificate on its own does not say whether an instrument is acceptable; the lab decides that by verifying the results against its own tolerances.
Is calibration the same as adjustment?
No. Calibration measures and documents how an instrument performs. Adjustment changes the instrument so its readings move closer to the correct values. A calibration provider may adjust an instrument, but good practice is to record as-found results before adjustment and as-left results afterwards, so you can assess the impact of any error on earlier work.
Does ISO/IEC 17025 specify calibration intervals?
No. ISO/IEC 17025:2017 requires a calibration programme that is reviewed and adjusted as necessary, but it does not set fixed intervals. Labs set intervals based on manufacturer guidance, frequency of use, criticality, environmental conditions and calibration history. Some accreditation bodies publish guidance, so check what yours expects for your scope.
Can a laboratory calibrate its own equipment?
Yes, provided it can demonstrate competence, metrological traceability and an evaluation of measurement uncertainty for that calibration. Many labs calibrate working thermometers against a calibrated reference thermometer, for example. Your accreditation body may assess in-house calibration as part of your assessment, so confirm its requirements before relying on it.
What is an intermediate check?
An intermediate check, covered by ISO/IEC 17025:2017 clause 6.4.10, is a planned check between calibrations to confirm equipment is still performing within requirements. Examples include a daily certified weight check on a balance or comparing an incubator thermometer with a reference. It must follow a procedure, use calibrated reference items and have defined acceptance limits.
Next steps
If you are preparing for accreditation or managing equipment in a testing lab, our ISO/IEC 17025 Advanced: Laboratory Competence & Accreditation course covers equipment, traceability and validity of results in depth. For related reading, see measurement uncertainty in testing labs and ISO 17025 accreditation: a practical roadmap. You can also speak to a counsellor about the right course for your role.
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