Calibration

Calibration vs Verification vs Adjustment: What Is the Difference?

Calibration, verification, and adjustment are three related but distinct activities in instrument management, and confusing them leads to compliance gaps and quality risks. This guide clarifies what each term means, how they relate to each other, and which is required for different regulatory and quality management contexts in Singapore.

Unitest Instruments·Updated 27 June 2026·9 min read

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.

This guide explains each term, illustrates the differences with practical examples, and clarifies how they apply in common Singapore industrial and regulatory contexts.

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), and by how much. The output of calibration is information — specifically, the errors and measurement uncertainties at each calibration point. Calibration does not, by definition, include any action to correct those errors. It is purely a measurement and reporting activity.

This is why a calibration certificate (from an accredited laboratory like Unitest Instruments under SAC-SINGLAS LA-2023-0845-C) reports measurement results and uncertainty — it tells you what the instrument actually does, not what you want it to do. What you do with that information — accept the instrument, 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."

In the context of measuring instruments, verification is the process of checking whether an instrument meets a specified requirement — typically a specification, a tolerance, or a performance standard. Verification uses the results of calibration (or a simplified check) and compares them against the acceptance criterion to reach a pass/fail conclusion.

A verification can be performed without a full calibration (for example, checking a thermometer against a known ice-water bath to confirm it reads 0°C ± 1°C), or it can follow a calibration (comparing the calibration results against the instrument's specification to determine whether it passes). The difference from calibration is that verification answers the binary question "does this instrument meet the requirement?" whereas calibration answers the quantitative question "what are the errors in this instrument's readings?"

Verification in legal metrology: In Singapore, the Weights and Measures Act and its subsidiary legislation require verification of certain weighing and measuring instruments used in trade (retail scales, petrol pump meters, etc.) by authorised verifiers under the authority of A*STAR's Standards, Productivity and Innovation Board (previously SPRING). This legal metrology verification is distinct from quality-system calibration — it is a statutory activity 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 physical or software action taken to reduce or eliminate errors identified by calibration. It is the corrective action, not the measurement. Adjustment might involve turning a zero-adjust screw on a mechanical pressure gauge, resetting the span on a temperature transmitter via HART communication, trimming the offset of a digital thermometer, or reconfiguring the internal calibration of a power analyser.

Critically, adjustment must be followed by calibration — you cannot know whether the adjustment was successful or by how much it improved the instrument's performance without performing a calibration after the adjustment. A calibration certificate that reports only pre-adjustment results does not confirm the instrument's state after adjustment.

How Calibration, Verification, and Adjustment Relate

The typical sequence when an instrument is submitted to a calibration laboratory is:

Initial calibration (as-found): The instrument is calibrated in its received condition, with no adjustments. The as-found results show how the instrument was performing when it arrived — this is valuable data for your quality system (did the instrument drift significantly since its last calibration?).
Verification: The as-found calibration results are compared against the instrument's specification (or your required tolerance). If within specification, the instrument passes and no adjustment is needed.
Adjustment (if required): If the as-found results show the instrument is out of its specification, adjustments are made to correct the errors.
Post-adjustment calibration (as-left): After adjustment, the instrument is calibrated again to confirm that the adjustment was successful and the instrument now meets its specification. The as-left results are what is reported on the final calibration certificate.

A calibration certificate that shows both as-found and as-left results provides the most complete picture of the instrument's history and the laboratory's work. When reading a certificate, check whether the results shown are as-found (pre-adjustment), as-left (post-adjustment), or both. For more on reading certificates, see our article on how to read a calibration certificate.

Common Confusion Between the Terms in Practice

Several common situations arise where these terms are confused or used incorrectly, leading to quality system gaps.

Using "Verified" When "Calibrated" Is Required

ISO 9001:2015 clause 7.1.5 states that monitoring and measuring resources must be "calibrated or verified" at specified intervals against measurement standards traceable to international or national measurement standards. Some quality managers interpret "verified" as meaning they can perform a simple functional check (e.g. measuring a known reference value with the instrument) in place of a full traceable calibration. This is only acceptable if the verification is performed with a reference standard that is itself traceable, the results are recorded, and the acceptance criterion is defined and documented. A check against an uncalibrated reference, or a visual inspection without measurement, does not satisfy the traceability requirement of ISO 9001 clause 7.1.5 in a metrologically rigorous sense.

Assuming Calibration Includes Adjustment

Some users submit an instrument for calibration expecting to receive it back in its best possible condition. If the laboratory performs calibration only (without adjustment), the instrument is returned with a certificate showing its errors, but those errors have not been corrected. Users should specify to the calibration laboratory whether they want calibration only (reporting of errors), or calibration with adjustment (reporting of errors plus correction). Unitest Instruments discusses service options with customers at the point of enquiry to ensure the right scope is agreed before calibration begins. Contact our team for advice on service options.

Treating a Calibration Label as Evidence of Calibration

A calibration sticker on an instrument shows that it was calibrated, but the sticker alone is not the calibration certificate. The certificate is the technical document that records the actual measurement results, reference standards, and uncertainty. Quality system auditors and regulatory inspectors will ask to see the certificate, not the sticker. See our guide on how to read and understand a calibration certificate for the full requirements.

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 using their own reference standards, rather than sending every instrument to an external accredited laboratory. This is acceptable under ISO 9001 and many other frameworks, provided:

The in-house reference standards are themselves calibrated by an accredited laboratory (such as Unitest Instruments under SAC-SINGLAS accreditation LA-2023-0845-C)
The in-house calibration procedure is documented and validated
The personnel performing in-house calibration are competent for the task
Records of in-house calibrations are maintained
The uncertainty of the in-house calibration is appropriate for the measurement requirement

For many organisations, a practical approach is to calibrate key reference standards externally at an accredited laboratory, and use those reference standards for in-house verification of working instruments in between external calibration events. This combines the traceability assurance of external accredited calibration with the convenience and cost efficiency of in-house verification. Unitest Instruments can supply and calibrate reference standards for use in in-house calibration programmes — visit our products page or calibration services page for more information.

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.

Frequently asked questions

Does ISO 9001 require calibration or verification, and what is the difference in practice?

ISO 9001:2015 clause 7.1.5 requires that monitoring and measuring equipment be "calibrated or verified" at specified intervals against measurement standards traceable to international or national standards. In practice, calibration (comparison against a traceable reference with recorded results and uncertainty) is the stronger form of evidence and satisfies the requirement unambiguously. Verification is acceptable but must still involve a traceable reference standard and documented acceptance criteria — a simple functional check against an unknown reference does not satisfy the traceability requirement. For safety-critical measurements or regulated industries, external accredited calibration is generally the safest approach.

If my instrument has been adjusted, does the old calibration certificate remain valid?

No — once an instrument has been adjusted, the pre-adjustment calibration certificate no longer represents the instrument's current state of calibration. Adjustment changes the instrument's readings, so the errors documented on the pre-adjustment certificate are no longer applicable. After any adjustment, the instrument must be calibrated again (as-left calibration) to determine its current errors and uncertainty. Only the post-adjustment calibration certificate is valid for quality system and regulatory purposes following an adjustment.

What is "as-found" and "as-left" calibration, and when should I ask for both?

As-found calibration records the instrument's performance in its received state, before any adjustment. As-left calibration records the instrument's performance after any adjustment has been made, in the condition in which it leaves the laboratory. Both are valuable: the as-found result tells you whether the instrument was in specification when it was in service (important for assessing measurements made since the last calibration); the as-left result confirms the instrument is in specification when returned to service. Request both as-found and as-left results from your calibration laboratory, especially for instruments used in regulated applications where measurement history matters.

Can I perform verification in-house instead of external calibration for all my instruments?

In-house verification is acceptable under ISO 9001 and many other frameworks, but only when performed with a reference standard that has a current traceable calibration certificate from an accredited laboratory, the verification procedure is documented and the acceptance criteria are defined, and competent personnel perform the verification. For instruments used in regulated applications (HSA GMP, legal metrology, SIL-rated safety systems), the requirements for documentation and traceability are more stringent, and external accredited calibration is typically expected. In-house verification is most appropriate for working instruments checked against in-house reference standards that are externally calibrated.

What is "calibration with tolerance check" and is it the same as calibration?

Calibration with tolerance check (sometimes called calibration with conformance statement) combines a calibration (measurement comparison) with a verification (comparison of results against a specification). The laboratory performs the full calibration measurements and then assesses whether the results fall within the instrument's specification or a user-specified tolerance, adding a pass/fail statement to the certificate. It is not a substitute for calibration — it is calibration plus an additional assessment. The measurement results and uncertainty must still appear on the certificate. This combined service is useful when you need both the quantitative measurement data (from calibration) and a formal compliance declaration (from the tolerance check).

Calibration, Adjustment, or Both — We Can Help

Unitest Instruments offers as-found and as-left calibration with adjustment under SAC-SINGLAS accreditation.