Calibration, Verification and Adjustment Are Not the Same Thing

Three activities with different outputs

Calibration, verification and adjustment are often recorded as though they were interchangeable steps in equipment control. They are not. Each answers a different technical question:

• Calibration determines the relationship between an instrument indication and a reference quantity value, under stated conditions.
• Verification establishes whether specified requirements have been met.
• Adjustment changes an instrument so that its indications better correspond to prescribed values.

The distinction matters because each activity produces different evidence. A calibration result is not automatically a pass decision. An adjustment is not evidence of performance before adjustment. A verification may be adequate for a defined operational purpose but may not provide the measurement information needed to establish traceability or evaluate measurement uncertainty.

Using the terms precisely prevents unclear certificates, incorrect equipment labels and unsupported assumptions about the validity of earlier or later results.

Calibration establishes measurement information

The International Vocabulary of Metrology, or VIM, defines calibration as an operation that, under specified conditions, establishes a relationship between quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties. That relationship can then be used to obtain a measurement result from an indication.

In practical laboratory work, calibration commonly produces one or more of the following:

• indication error or correction at specified points;
• repeatability or other observed performance characteristics;
• an uncertainty statement for the reported result;
• the conditions, standards and method under which results were obtained.

Consider a 0 bar to 10 bar pressure indicator calibrated at 2 bar, 5 bar and 8 bar. At 5 bar, the reference pressure is 5.000 bar and the instrument indicates 5.060 bar. The calibration result may be reported as an indication error of +0.060 bar, with an associated expanded uncertainty. That is measurement evidence. It does not by itself say whether the indicator is suitable for its intended use.

Suitability depends on the applicable specification, the decision rule and the risk accepted by the user. If the permissible error is ±0.10 bar, a conformity decision may be possible. If the permissible error is ±0.05 bar, the same result may be unsatisfactory. In either case, the calibration data remain the underlying result.

Calibration also does not necessarily mean that every performance attribute has been evaluated. A calibration procedure must be appropriate to its stated purpose. A single-point check may establish a correction at one point, but it does not demonstrate performance across the full range.

Verification reaches a requirement-based decision

Verification is the provision of objective evidence that an item fulfils specified requirements. The requirement must therefore be defined before verification can be meaningful.

For a balance used to prepare a single nominal mass, verification may mean checking that error at that operating point remains within a locally defined acceptance limit. For a temperature-controlled chamber, verification may involve demonstrating that measured temperature variation and offset meet a process requirement at the locations and setpoints actually used.

Verification can use calibration data, intermediate-check data or a dedicated test. Its essential output is normally a status decision such as accepted, rejected, restricted for use, or requiring further evaluation.

A verification record should make clear:

• the requirement or acceptance limit applied;
• the method and reference equipment used;
• the observed result and, where relevant, its uncertainty;
• the decision rule used where measurement uncertainty affects the acceptance decision;
• the resulting equipment status and any use restrictions.

This distinction is particularly important when a laboratory writes “calibrated and passed” on an internal record. That phrase combines two separate statements: a calibration result was obtained, and a conformity decision was made. Both may be valid, but the record should preserve the basis for each.

ISO/IEC 17025:2017 requires laboratories to verify equipment when necessary to ensure it meets specified requirements before being placed or returned into service. This is not a requirement to verify every item in the same way. The required verification depends on the equipment, its intended use and the risk that unsuitable performance would create.

Adjustment changes the instrument

Adjustment is a set of operations carried out on a measuring system so that it provides prescribed indications corresponding to given values of a quantity being measured. Examples include zeroing a pressure transmitter, trimming a temperature transmitter offset, or changing the span setting of an analogue input module.

Adjustment changes the item. Calibration characterises the item.

An instrument may be adjusted as part of maintenance, after a failed verification, or as an intentional process step before use. However, adjustment creates an immediate control question: what evidence supports the instrument's performance after the change?

A pre-adjustment calibration remains important evidence of the condition found. It may support an assessment of whether earlier measurement results could have been affected. It does not establish post-adjustment performance. Following adjustment, the organisation should perform an appropriate calibration, verification or both before returning the item to service.

The extent of post-adjustment testing should be technically justified. A zero adjustment on a limited-range indicator may warrant confirmation at zero and one or more working points. A span adjustment, firmware-related configuration change or repair affecting the measurement chain may require a fuller recalibration across the intended range.

Records should state clearly whether results are as found, as left, or both. Without that distinction, a later reviewer cannot determine whether reported errors represent the instrument received from service or the instrument after intervention.

Equipment status must follow the activity performed

Equipment labels and database status should describe the actual control state, not use generic language that obscures it. “Calibrated” may indicate that a calibration event occurred, but it does not necessarily mean the equipment has been accepted for a particular application. “Verified for use from 0 °C to 50 °C” is more informative when verification has been limited to that range and purpose.

Where an item is adjusted following an out-of-tolerance result, it should remain controlled from the point of discovery until post-adjustment evidence supports release. The technical review should also consider the period since the last satisfactory evidence, the magnitude and direction of error, the measurement decisions affected and any available intermediate-check history. A failed result does not automatically invalidate all prior work, but it does require a documented, evidence-based assessment.

Clear terminology is a practical discipline rather than a semantic preference. Calibration provides measurement information. Verification compares evidence against requirements. Adjustment changes performance. Recording those activities separately preserves what happened to the equipment, supports defensible release decisions and ensures subsequent users understand the status of the measurements they rely upon.

References

• JCGM 200:2012, *International Vocabulary of Metrology - Basic and General Concepts and Associated Terms (VIM)*, 3rd edition, BIPM.
• ISO/IEC 17025:2017, *General requirements for the competence of testing and calibration laboratories*, particularly clauses 6.4 and 7.8.
• ILAC G8:09/2019, *Guidelines on Decision Rules and Statements of Conformity*.