Calibration and adjustment are two different operations — and confusing them is one of the most common mistakes in quality management systems. Calibration is a measurement process: it documents how closely an instrument agrees with a traceable reference standard. Adjustment is a corrective action: it physically or electronically alters the instrument to reduce that error. You can calibrate without adjusting, but you should never adjust without calibrating first.
This distinction isn’t just semantic. ISO/IEC 17025:2017 and the International Vocabulary of Metrology (VIM) treat them as separate operations with different documentation requirements. In regulated industries — aerospace, pharmaceutical, medical device manufacturing — using these terms interchangeably can produce audit findings and compromise the integrity of your measurement records.
This article explains exactly what each term means according to international standards, how they interact in a complete calibration service, when adjustment is warranted (and when it isn’t), and what your calibration certificates should show when both operations are performed.
Key Takeaways
- Calibration is a measurement-only process — it compares an instrument’s output to a traceable standard and documents the deviation without physically changing the instrument.
- Adjustment is a corrective action — it alters the instrument to reduce the error found during calibration. Formally, per the BIPM VIM (clause 3.11), adjustment is a separate operation from calibration.
- A complete service cycle includes “as found” data (before any adjustment), the adjustment itself (if performed), and “as left” data (after adjustment) — all documented on the calibration certificate.
- Not every in-tolerance instrument requires adjustment. Unnecessary adjustments can reset drift baselines and complicate long-term performance tracking.
What Is Calibration, Exactly?
Calibration is a two-step operation defined in the BIPM’s International Vocabulary of Metrology (VIM, clause 2.39): it first establishes a relation between quantity values provided by measurement standards (with stated uncertainty) and the instrument’s corresponding indications; it then uses that relation to derive measurement results from future indications. The operative word is establishes — calibration is observation, comparison, and documentation. It does not, by definition, include any physical change to the instrument.
The output of a calibration is data. Specifically, a calibration certificate showing how the instrument performed against a traceable reference at the time of service, including the measurement uncertainty associated with those results. That data becomes part of your quality record — usable for drift analysis, calibration interval decisions, and demonstrating metrological traceability during audits under ISO 9001, AS9100, or FDA 21 CFR Part 820.
Traceability is the critical thread. For calibration data to be meaningful, the reference standard used must itself be calibrated — and that standard’s calibration must trace back, through an unbroken chain of comparisons, to national or international measurement standards. Without that chain, the numbers on a certificate are measurements relative to an unknown reference, not relative to reality.
For a deeper explanation of what calibration documentation must include and who is authorized to issue it, see our guide to calibration certificates explained.
What Is Adjustment in Metrology?
Adjustment is defined in VIM clause 3.11 as “a 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.” In plain terms: someone physically or digitally modifies the instrument to bring its output into agreement with the known reference value. Adjustment changes the instrument; calibration only observes it.
Adjustments come in three forms. Mechanical adjustment involves turning zero and span screws or moving physical components — common on pressure gauges, dial indicators, and analog panel meters. Electronic adjustment trims the analog signal path — typical in process transmitters and signal conditioners. Software or digital offset adjustment applies a correction factor in firmware or calibration software — used in smart instruments, data loggers, and digitally compensated sensors.
Regardless of method, one rule applies universally: after any adjustment, the instrument must be re-verified. You cannot assume the adjustment landed where you intended it. The post-adjustment verification measurement is what produces the “as left” data on a calibration certificate, confirming the instrument now meets its specified tolerance before it returns to service.
It’s worth noting that informal shop-floor language often collapses both operations into a single word: “calibrate” used to mean “go adjust it.” That usage is technically incorrect and creates real documentation gaps in audited environments. A certificate showing only post-adjustment data tells you where the instrument ended up, not where it was during the period it was generating production measurements.
Calibration vs. Adjustment: The Core Differences
The table below summarizes the operational differences. These distinctions matter when writing calibration procedures, interpreting certificates, or responding to an audit nonconformance about your measurement control system.
| Dimension | Calibration | Adjustment |
|---|---|---|
| Definition (VIM) | Establishes relation between standard values and instrument indications (VIM 2.39) | Operations to make an instrument provide prescribed indications (VIM 3.11) |
| Changes the instrument? | No — observation and documentation only | Yes — physically or digitally alters the instrument |
| Output | Calibration certificate with measurement data and uncertainty | Adjusted instrument; documented in the “as left” section of the certificate |
| Required by ISO/IEC 17025? | Yes — primary deliverable of an accredited calibration lab | Only when the customer requests it, or the instrument is out of tolerance |
| Generates uncertainty data? | Yes — required per ISO/IEC 17025:2017 clause 7.8.4 | Not directly — uncertainty applies to the measurements, not the adjustment act itself |
| Sequence | Always first — you calibrate to find the error before deciding whether to adjust | Always after calibration — adjustment without prior measurement is guesswork |
| Can it stand alone? | Yes — calibration-only service produces a valid certificate | No — must be preceded by calibration (as-found) and followed by verification (as-left) |
The As-Found / As-Left Workflow: How Calibration and Adjustment Work Together
In practice, a complete calibration service at an ISO/IEC 17025-accredited laboratory follows a structured four-phase sequence. Understanding this workflow is essential for interpreting calibration certificates correctly and for setting up your internal calibration procedures.
According to Withnell Sensors and major calibration providers, the complete workflow includes:
Phase 1: As-Found Measurement
Before any work is done to the instrument, a technician measures it against the traceable reference standard across its full operating range (or the specified calibration points). This captures the instrument's actual performance during the period it was in service — reflecting real-world drift, environmental exposure, and usage since the last calibration. The as-found data tells you whether the instrument was in tolerance before it was serviced.
Phase 2: Tolerance Evaluation
The as-found data is compared to the instrument's specified tolerance. If every measurement falls within tolerance, the calibration is complete — no adjustment is needed or warranted. The certificate documents the as-found results and the instrument returns to service. If any measurement falls outside tolerance, the service moves to Phase 3.
Phase 3: Adjustment (Out-of-Tolerance Only)
The technician modifies the instrument (mechanically, electronically, or via software) to reduce the measured error. After adjustment, the instrument is re-measured. In many regulated quality systems, this step also triggers an out-of-tolerance (OOT) notification — because if the instrument was out of tolerance during its last service period, any measurements it generated during that period are suspect and may need to be reviewed.
Phase 4: As-Left Measurement
A final verification confirms the instrument's performance after adjustment, documenting where it was "left" for the next calibration cycle. Both as-found and as-left data sets appear on the calibration certificate. If no adjustment was performed, only as-found data is recorded (and it serves as both the as-found and as-left result).
When Should You Adjust — and When Shouldn’t You?
The decision to adjust isn’t automatic. If a calibration reveals that an instrument is within its specified tolerance, adjusting it can introduce more risk than it eliminates — and this is a point that surprises many quality engineers when they first encounter it.
Process instrumentation experts at Beamex are direct on this point: do not make unnecessary adjustments, because every adjustment makes drift analysis more difficult. Here’s why that matters:
Drift Trending Depends on Consistent As-Found Data
Drift trending is the practice of watching how an instrument’s error changes between calibrations. It lets you predict when an instrument will go out of tolerance, which in turn lets you optimize your calibration interval — extending intervals for stable instruments and shortening them for instruments with high drift rates. This analysis depends on comparing as-found data across multiple calibration cycles. Every time you adjust an in-tolerance instrument, you reset the drift baseline to zero. The trend data becomes discontinuous, and the predictive value of your historical records is lost.
Unnecessary Adjustment Introduces New Error Opportunities
Every mechanical adjustment involves handling the instrument and manipulating its internal components. Each interaction is an opportunity to introduce hysteresis, introduce contamination, or inadvertently alter the span setting while correcting the zero. Digital adjustments carry their own risk: entering incorrect correction factors, saving to the wrong memory location, or triggering unintended parameter resets. If the instrument was in tolerance before you touched it, there’s no quality-system reason to take on those risks.
Decision Rule for Adjustment
A practical rule of thumb: adjust when the as-found data shows an out-of-tolerance condition. Some quality systems set a more conservative threshold — for example, adjusting any instrument that reads more than 80% of its tolerance band to avoid the risk of it drifting OOT before the next calibration cycle. Whatever threshold you use, it should be documented in your calibration procedure so the decision is traceable and consistent across technicians.
When not to adjust: when the instrument is well within tolerance and drift history doesn’t suggest it will go OOT before the next scheduled calibration. If your tolerance is ±1% of span and the instrument reads -0.25% of span, adjustment creates risk without delivering any quality benefit. Understanding how calibration intervals are set using risk-based criteria helps you make this decision with confidence.
What Your Calibration Certificate Should Show
ISO/IEC 17025:2017 clause 7.8 specifies what calibration certificates must contain. When adjustment was performed, the certificate must clearly distinguish between what was found and what was done. A compliant certificate should include all of the following:
- As-found data — the instrument’s readings before any adjustment, at each calibration point
- Notation that adjustment was performed — including the type of adjustment (zero, span, offset, etc.)
- As-left data — the instrument’s readings after adjustment, confirming it was returned to service within tolerance
- Measurement uncertainty — stated for both the reference standard and the calibration results (clause 7.8.4)
- Reference standard identification — the standard used, with its own calibration certificate number and traceability chain
- Environmental conditions — temperature, humidity, and any other conditions that could affect the measurement
- Calibration date and technician — for traceability to the specific service event
If a certificate only shows one set of data for an instrument that was adjusted, ask the issuing lab for the complete record. Under ISO/IEC 17025:2017, accredited laboratories are required to maintain complete calibration records. The as-found data is part of that record.
Not all calibration certificates are created equal. Understanding the difference between working and reference standards also helps you evaluate the traceability chain behind any calibration certificate you receive.
Why This Distinction Matters for Compliance
In regulated quality environments, the calibration-vs-adjustment distinction has real audit and corrective action implications. Each major standard addresses it differently, but all share the same underlying principle: what the instrument was doing during its service period must be documented separately from what was done to bring it back into spec.
ISO 9001:2015 clause 7.1.5 requires organizations to provide monitoring and measuring resources that are fit for purpose and to maintain them. Calibration records must demonstrate traceability. A record that conflates calibration and adjustment — showing only adjusted values without the pre-adjustment baseline — does not demonstrate that the instrument was in a known state during production.
ISO/IEC 17025:2017 governs accredited calibration laboratories directly. Clause 7.8.2 requires certificates to accurately reflect the calibration scope actually performed. If adjustment was performed and only post-adjustment data appears, the certificate misrepresents the scope of the service.
FDA 21 CFR Part 820.72 (medical device QMS) requires that calibration records include “the calibration method used, the allowable tolerances, the actual results obtained” — all of which imply as-found documentation separate from any subsequent adjustment.
AS9100 Rev D clause 7.1.5 goes further: instruments found out of tolerance must be followed by documented investigation into prior measurements. That investigation begins with the as-found data. Without it, you cannot determine whether product manufactured with that instrument was affected — which is the central question in any OOT corrective action.
The as-found data isn’t a quality-system formality. It’s the evidentiary record that determines whether a nonconformance investigation is needed. The gap between “what calibration found” and “what adjustment corrected” is, in regulated industries, the difference between a routine service event and a potential product recall trigger.
If your equipment requires NIST-traceable calibration with complete as-found and as-left documentation, our team issues certificates that meet ISO/IEC 17025:2017 requirements. Request a quote to discuss your calibration program.
FAQ
No. Calibration is a measurement process that compares an instrument’s output to a traceable reference standard and documents the results — it does not change the instrument. Adjustment is a separate corrective action that physically or digitally modifies the instrument to reduce the error found during calibration. The BIPM International Vocabulary of Metrology (VIM) defines them in separate clauses (2.39 and 3.11 respectively).
“As found” data is recorded before any adjustment is made — it shows the instrument’s actual condition at the start of the service. “As left” data is recorded after any adjustment — it confirms the instrument is within tolerance before it returns to service. If no adjustment was made, only as-found data is recorded, and it serves as both the before and after result.
Generally, no. Adjusting an in-tolerance instrument resets its drift baseline, which complicates long-term trend analysis used to optimize calibration intervals. It also introduces unnecessary handling risk. Best practice is to document the in-tolerance as-found result and return the instrument to service without adjustment. Some organizations set internal thresholds (e.g., adjust if reading exceeds 80% of tolerance band) — these should be documented in the calibration procedure.
An out-of-tolerance (OOT) finding triggers a documented investigation under most quality systems (ISO 9001, AS9100, FDA 21 CFR Part 820). You must assess whether measurements taken with that instrument during its previous service period were affected, and whether any product or test data needs to be reviewed. The as-found data from the calibration is the starting point for that investigation. See our guide on out-of-tolerance calibration procedures for the required corrective action steps.
In-house adjustment is permissible under ISO 9001 and ISO/IEC 17025 if your internal process is documented, your technicians are trained, and the reference standard used for the post-adjustment verification is itself calibrated and traceable. However, the adjustment and all associated measurements must be documented in your calibration records. For instruments requiring accredited calibration certificates — typically specified by customers or regulatory bodies — adjustment must be performed by, or coordinated with, an ISO/IEC 17025-accredited laboratory.
The as-found data across multiple calibration cycles is the primary input for drift-based interval analysis. If an instrument consistently returns well within tolerance, your intervals can often be extended. If it consistently returns near or beyond tolerance limits, intervals should be shortened — or the instrument investigated for an underlying cause (environmental stress, mechanical wear, inappropriate application). Adjusting in-tolerance instruments disrupts this trend data. See our article on calibration interval determination for a risk-based approach to setting intervals.
The Short Version
Calibration measures. Adjustment corrects. They’re related but distinct operations — one documents the instrument’s state, the other changes it. You need calibration data (as-found) before you can justify any adjustment, and you need post-adjustment verification (as-left) before you can trust the result. In regulated industries, keeping these operations separate in your documentation isn’t just best practice — it’s what makes your quality records defensible.
If you’re building or reviewing a calibration management program, understanding this distinction early saves significant rework down the line. Related reading: calibration vs. verification vs. validation, and 10 questions to ask your calibration service provider before signing a service agreement.
For instruments requiring NIST-traceable calibration with complete as-found and as-left documentation, contact Micro Precision to discuss your requirements.