For quality managers: when to calibrate vs adjust, the as found to as left workflow, and how to preserve traceability to avoid audit findings.
Calibration measures how far an instrument’s readings deviate from a traceable standard, while adjustment physically changes the instrument to correct that deviation. The correct sequence is always calibrate first, decide whether adjustment is warranted, then adjust and recalibrate to confirm the fix worked. Skipping straight to adjustment without a documented baseline puts your measurement traceability and your audit trail at risk.
TL;DR:
- Calibrating an instrument first captures its initial performance, providing a traceable baseline needed to determine if adjustment is truly necessary.
- Documented as-found and as-left readings, along with measurement uncertainty, are essential for audit compliance and for proving adjustments corrected existing faults.
- Adjustment should only be performed after calibration confirms the deviation exceeds permissible error, preventing unnecessary changes and maintaining traceability.
- Regular calibration intervals must be based on instrument stability, criticality of measurement, environment, and manufacturer recommendations, not just fixed schedules.
- Ignoring proper sequencing or missing records during calibration increases audit risks, potential non-conformities, and undermines measurement confidence over time.
Getting the vocabulary right matters more than it seems, because auditors and quality managers rely on these terms meaning exactly one thing each. The International Vocabulary of Metrology defines calibration as the operation that establishes a relation between the values from a measurement standard and the corresponding indications of an instrument, along with the associated measurement uncertainty. Calibration does not change the instrument. It documents how the instrument is currently performing.
Adjustment sits on the other side of that line. The VIM describes adjustment of a measuring system as the set of operations carried out to make a measuring system provide prescribed indications for given values of a quantity being measured. Adjustment changes the device itself, whether through a physical tweak, a software offset or a recalibration constant.
A few related terms are worth locking down as well:
Calibrating before adjusting is not a formality. It is the only way to know whether an instrument actually needs correcting. Adjust first and you lose the as-found data that tells you the size and direction of the original error, and you risk chasing noise rather than fixing a real fault.
NATA’s guidance on ISO/IEC 17025 stresses that accredited facilities must document calibration ranges, measurement uncertainty and the evidential basis for every result, including proficiency testing where relevant. This is not paperwork for its own sake. It is what lets a manufacturer or lab prove, months or years later, that a batch of product or a set of test results was measured correctly.
A common non-conformance in accreditation audits is missing as-found or as-left records after an adjustment, according to NATA’s ISO/IEC 17025 presentation. That single gap can trigger corrective action requests and cast doubt on every measurement taken since the last known-good calibration.
When an instrument is found out of tolerance, the correct response is a documented impact assessment and, where warranted, root-cause analysis, before any physical adjustment takes place.
Not every out-of-tolerance result calls for adjustment. Technicians should weigh the size of the deviation against the maximum permissible error for that instrument’s application, the stated uncertainty of the calibration itself, how critical the measurement is to product safety or compliance, and the instrument’s drift history over previous calibrations.
A few examples make the criteria concrete:
Pro Tip: Always record the as-found reading before touching a single screw or software setting, even when the fault looks obvious.
Once adjustment is chosen, the technician should isolate the instrument from production use, perform the adjustment, and immediately schedule a fresh calibration rather than assuming the fix worked.
A defensible calibration and adjustment process follows a fixed sequence: calibrate to capture as-found data, decide whether the deviation warrants action, adjust or repair if needed, recalibrate to produce as-left data, and complete an impact assessment if the as-found result was out of tolerance. This is the sequencing workflow instrumentation providers and metrology bodies point to when explaining why adjustment and calibration are separate operations, never one combined step.
Every calibration record, regardless of instrument type, should capture:
These fields are exactly what appears on a proper calibration certificate, and they are what an auditor checks first. Without as-found and as-left data side by side, there is no way to prove an adjustment actually corrected the problem it was meant to fix.
Terminology slips cause more audit trouble than most quality managers expect. A few patterns show up repeatedly:
Undocumented adjustments are the clearest red flag of all. If nobody wrote down that an instrument was touched between two calibrations, every result measured with it in between becomes questionable.
Fixed calendar intervals are a starting point, not a rule. A defensible interval reflects the instrument’s actual behaviour and its role in your operation.
Practical calibration frequency guidance is worth reviewing when you need to justify a deviation from a standard schedule, since auditors expect that decision to be evidence-based rather than arbitrary.
The mistake we encounter most is treating adjustment as the first step rather than the last. A technician notices a reading looks off, adjusts on the spot, and only then wonders whether the instrument actually needed it. By that point the as-found evidence is gone. Our calibrations generate the as-found and as-left record an auditor will eventually ask to see. Getting that sequence right the first time saves far more trouble than fixing it after an audit finding.
— Nima
Getting the sequence right on your own equipment is easier with a provider who documents every step as standard. We offer calibration and trade verification services that include the as-found and as-left records an auditor expects as part of the paperwork you receive.
Before you contact us, have your instrument’s serial number, its last calibration date and any recent service history ready. That lets us scope the job accurately and flag whether adjustment is likely before a technician arrives.
Visit our calibration services page to request a quote or discuss on-site versus workshop calibration for your equipment.
For primary definitions, see the VIM entries on calibration and adjustment, NATA’s ISO/IEC 17025 application guidance, and this industry explainer on calibration terminology.
ISO 9001 requires organisations to ensure monitoring and measuring equipment is suitable for its purpose and to keep evidence supporting the validity of results, including calibration records and traceability. ISO’s auditing practices guidance notes that auditors expect calibration intervals and methods to be chosen based on risk and recorded performance rather than a fixed calendar alone.
Calibration is commonly grouped into internal calibration, performed in-house against a lab’s own reference standards, and external or accredited calibration, performed by a body such as a NATA-accredited provider against nationally traceable standards. NATA’s application guidance notes that in-house calibration can be assessed for fitness for purpose, but accredited calibration carries the stronger traceability claim.
Calibration means checking an instrument’s readings against a known, traceable standard and recording exactly how far off it is, without changing the instrument. The result is a certificate showing the deviation and its measurement uncertainty, as defined in the VIM.
There is no exact synonym, since calibration has a specific technical meaning distinct from related terms like verification, adjustment and tuning. Verification is the closest in everyday use, but it only confirms pass or fail against a requirement, while calibration quantifies the actual deviation and its uncertainty.
