Calibration, verification, and adjustment are three distinct activities in instrument management. Calibration determines errors, verification confirms whether those errors are acceptable, and adjustment corrects the instrument to reduce errors, and they are often confused, leading to compliance gaps in quality management systems. ISO/IEC 17025:2017 defines these terms clearly, and understanding the distinctions is essential for quality engineers, laboratory managers, and calibration programme managers in Singapore who must satisfy auditors, regulators, and customers that their measurement equipment is properly managed.
Defining Calibration
The international vocabulary of metrology (VIM, JCGM 200:2012) defines calibration as: "Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication." In plain terms: calibration is the measurement comparison that determines how much an instrument's readings differ from the true value represented by a traceable reference standard. The output is information — the errors and uncertainties at each point — not a correction. This is why a certificate from an accredited laboratory like Unitest Instruments (SAC-SINGLAS LA-2023-0845-C) reports results and uncertainty: it tells you what the instrument actually does. What you do with that information — accept it, adjust it, or retire it — is a separate decision.
Defining Verification
The VIM defines verification as: "Provision of objective evidence that a given item fulfils specified requirements." For measuring instruments, verification checks whether an instrument meets a specified requirement (a specification, tolerance or performance standard), using calibration results or a simplified check compared against an acceptance criterion to reach a pass/fail conclusion. Calibration answers "what are the errors?"; verification answers "does this instrument meet the requirement?" In Singapore, the Weights and Measures Act requires statutory verification of certain trade instruments (retail scales, petrol pump meters) by authorised verifiers under A*STAR — distinct from quality-system calibration, with specific legal consequences for non-compliance.
Defining Adjustment
The VIM defines adjustment as: "Set of operations carried out on a measuring system so that it provides prescribed indications corresponding to given values of a quantity to be measured." Adjustment is the corrective action, not the measurement — turning a zero-adjust screw on a mechanical gauge, resetting a transmitter's span via HART, trimming a digital thermometer's offset, or reconfiguring a power analyser's internal calibration. Critically, adjustment must be followed by calibration: you cannot know whether it was successful without measuring again afterward. A certificate reporting only pre-adjustment results does not confirm the instrument's post-adjustment state.
How They Relate: The Typical Sequence
When an instrument is submitted for calibration: initial calibration (as-found) measures the instrument as received, with no adjustment — valuable data showing whether it drifted since last calibration; verification compares the as-found results against specification or tolerance to determine pass/fail; adjustment follows if the as-found results show the instrument out of specification; and post-adjustment calibration (as-left) confirms the adjustment succeeded, with these results reported on the final certificate. A certificate showing both as-found and as-left results gives the most complete picture — when reading one, check which is shown. See our article on how to read a calibration certificate.
Common Confusion Between the Terms in Practice
Using "verified" when "calibrated" is required: ISO 9001:2015 clause 7.1.5 allows equipment to be "calibrated or verified" against traceable standards. Some quality managers treat this as licence for a simple functional check, but this only satisfies the clause if performed with a traceable reference standard, recorded results, and a documented acceptance criterion — a check against an uncalibrated reference or a visual inspection does not. Assuming calibration includes adjustment: if a laboratory performs calibration only, the instrument returns with a certificate showing its errors uncorrected. Specify whether you want calibration only or calibration with adjustment before work begins. Treating a calibration label as evidence: a sticker shows calibration occurred, but it is not the certificate itself — auditors ask to see the certificate, not the sticker. See our guide on how to read a calibration certificate.
Which Term Applies in Singapore's Key Regulatory Frameworks?
| Regulatory Framework | Relevant Term | Specific Requirement |
|---|---|---|
| ISO 9001:2015 (clause 7.1.5) | Calibration or verification | Traceable calibration or verification at specified intervals; records retained |
| HSA GMP (pharmaceutical) | Calibration | Critical instruments calibrated against traceable standards; records maintained for GMP inspection |
| Weights and Measures Act (Singapore) | Verification (legal metrology) | Statutory verification of trade measuring instruments by authorised verifiers |
| MOM WSH Act | Maintenance (includes calibration) | Safety-critical measuring instruments maintained in safe and reliable condition |
| IEC 61511 (safety instrumented systems) | Proof testing (includes verification) | Periodic functional tests of safety instruments at SIL-appropriate intervals |
| ISO 14001 (environmental) | Calibration or verification | Monitoring equipment calibrated or verified; records maintained |
In-House Calibration and Verification vs External Accredited Calibration
Some organisations perform in-house calibration or verification with their own reference standards rather than sending every instrument externally. This is acceptable under ISO 9001 and similar frameworks provided the in-house reference standards are themselves calibrated by an accredited laboratory, the procedure is documented and validated, personnel are competent, records are maintained, and the uncertainty achieved is appropriate for the measurement requirement. A practical approach for many organisations: calibrate key reference standards externally at an accredited laboratory, then use those standards for in-house verification of working instruments between external calibration events, combining traceability assurance with convenience and cost efficiency. Unitest Instruments can supply and calibrate reference standards for in-house programmes — visit our products page or calibration services page.
Summary: Three Terms, Three Activities
- Calibration: The measurement comparison that determines what errors are present in an instrument. The output is a certificate reporting results and uncertainty. Does not correct errors.
- Verification: The assessment of whether calibration results (or a simpler check) meet a specified acceptance criterion. Answers "pass or fail?" Does not correct errors.
- Adjustment: The physical or software correction applied to reduce errors identified by calibration. Must be followed by calibration to confirm effectiveness. Changes the instrument's output.
Understanding these distinctions helps you specify the right service from your calibration provider, interpret certificates correctly, and structure your quality management system's instrument management procedures in a way that satisfies auditors and regulators. If you have questions about which service is right for your instruments, contact the Unitest Instruments team. We respond to enquiries within 2 business hours.