The distinction matters because each one answers a different challenge. A reviewer asks what the instrument’s relationship to the true value was, whether it was still within its limits when the test was performed, and whether anything in the chain outside the instrument changed. Those are three questions and three pieces of evidence.

Three different things called calibration

The first is calibration in the strict sense: a comparison against a reference that establishes the relationship between the instrument’s indication and the true value, expressed with an uncertainty. It is performed at a defined interval and produces a certificate.

The second is verification: a check that the instrument is performing within its stated limits. It may be performed in-house, it may use a reference of known value, and it produces a record rather than a certificate. Its purpose is to detect change between calibrations.

The third is the field or pre-use check: a functional confirmation that the instrument is operating, that its battery and connections are sound and that it responds as expected. It does not establish accuracy; it establishes that nothing has failed since the last verification.

Traceability and what it requires

Traceability is an unbroken chain of comparisons, each with a stated uncertainty, linking the instrument’s indication to a national or international reference. A certificate whose chain is broken at any point demonstrates that a comparison was made, not that the result is traceable.

The chain has two requirements that are commonly missing in practice. The first is that each link names the reference it was compared against. The second is that each link carries an uncertainty, because an unbroken chain without uncertainties cannot be quantified.

The reference framework for the units themselves is published by BIPM, and the practical guidance on establishing and documenting traceability for measurement equipment is described in the NIST handbooks. The accreditation framework that a calibration laboratory is assessed against is described by ILAC and IAF, and the requirements for legal metrology instruments are published by OIML.

In-house verification between calibrations

Verification between calibrations is what detects drift before it affects a result. The reference used has to be suitable for the quantity and stable enough that a change in the reading can be attributed to the instrument under test rather than to the reference.

The verification record should state the quantity checked, the reference used and its own calibration status, the reading obtained and the position within the instrument’s stated limits. A record that states only pass or fail loses the ability to detect a gradual move toward a limit, which is the information that allows the interval to be adjusted.

The interval between verifications should follow the instrument’s behaviour in that service. An instrument that has been stable for years can be verified less often than one that has shown movement, and the interval should be recorded with the reasoning so that a later reviewer can see why it was chosen.

Field checks that catch drift early

Field checks address the parts of the measurement chain that the calibration does not cover. A calibration certificate for an instrument says nothing about the leads, the clamps, the coupling capacitor or the connectors, and those are frequently where a discrepancy originates.

A practical field check includes a visual inspection of the leads and connectors for damage, continuity and corrosion; a check that the instrument’s own self-test or reference function behaves as expected; and, where the test permits, a measurement on a known reference such as a calibration resistor or a stable reference source.

The check should be recorded with the test, not only with the instrument. That way, a later reviewer sees that the chain was checked at the time of the measurement rather than inferred from a calibration certificate issued months earlier.

Intervals by instrument type and use

The calibration interval should follow the instrument’s stability, its exposure and the consequence of an incorrect result. A reference divider used to establish test voltages for acceptance testing deserves a shorter interval than a handheld instrument used for screening measurements.

The manufacturer’s recommended interval is the starting point. It should then be adjusted by the recorded history: an instrument that has never moved in five annual calibrations can be extended, and one that has moved appreciably can be shortened. Adjusting the interval on evidence is what prevents a fixed interval from being either wasteful or inadequate.

Instruments used in conditions outside their reference range deserve attention. A test set used in a hot substation in summer is operating away from the conditions under which its accuracy specification applies, and the manufacturer’s temperature coefficient becomes the relevant figure rather than the nominal accuracy.

Records a laboratory or client will accept

The record set that survives review contains, for each instrument, its identification and serial number, the calibration certificate with the traceability chain and uncertainty, the verification records between calibrations, the interval and the basis for it, and the instrument’s history of adjustment or repair.

Alongside the instrument records, the test report needs the chain information: the leads, dividers or coupling used, their identification and their calibration status where they are part of the measurement. The measurement system requirements that govern how a high-voltage measuring system is qualified are set out in IEC 60060-2.

The records should be held against the instrument rather than against the project, so that the history follows the instrument. A record set filed by project scatters the instrument’s history across documents, and the interval decision then has nothing to work from.

Handling an instrument that fails verification

An instrument that fails verification is withdrawn from service, labelled, and sent for calibration or repair. That part is straightforward and usually done.

The step that is often missed is the review of measurements taken since the last successful verification. If the instrument has been drifting, tests performed during the drift period may be unreliable, and the review determines which results need to be repeated or qualified.

The review should be recorded. An entry stating which tests were performed with the instrument during the period, and the decision taken for each, is what allows a client to be told about the situation rather than discover it later.

Building the calibration register

The register is the document that makes the process manageable. One row per instrument, with its identification, the location, the last calibration date and result, the next due date, the verification interval, the person responsible and the certificate reference.

The register should also record which instruments are part of a measurement chain that has to be calibrated as a chain, and which are standalone. That distinction determines whether an individual certificate is sufficient or whether a chain certificate is required, and it is easy to lose track of when the equipment list grows.

Finally, the register should link to the test records in which each instrument was used, at least for the instruments on which a decision depends. That link is what makes it possible to answer the question a client eventually asks: which measurements were performed with the instrument that has just been found to be out of calibration. Equipment service and support arrangements, including calibration support, are described under testing services and OEM/ODM solutions, the product range those instruments come from is listed under all electrical testing equipment, and the certification scope of the range is shown on the certifications page.