Calibration and Measurement Control

Chapter objectives

After this chapter, the reader should be able to:

• distinguish calibration, adjustment, verification, intermediate checking, maintenance and repair;
• identify which equipment requires measurement control and assign clear ownership;
• maintain an equipment register that connects each item to its intended measurement process;
• define calibration range, points, method, uncertainty and acceptance requirements before service;
• select a calibration provider by current accredited scope rather than by accreditation logo alone;
• prepare, protect and transport instruments without changing their condition;
• review a calibration certificate for identity, coverage, results, uncertainty, traceability and decision rule;
• interpret error, correction and uncertainty without treating them as interchangeable;
• establish and revise calibration intervals using risk, history and operating conditions;
• apply intermediate checks that detect change between calibrations;
• control equipment status, access, storage, handling and environmental exposure;
• contain overdue, damaged, failed or doubtful equipment and assess affected work;
• separate the measurement result, conformity statement and product-release authority;
• retain records that reconstruct the equipment life cycle; and
• troubleshoot common calibration and measurement-control failures.

The first control question is not “When is the instrument due?” It is “Which decision depends on this measurement, and what could happen if the result is wrong?”

Define:

1. the product, process or safety decision;
2. the characteristic and unit;
3. nominal value, tolerance or control limit;
4. required measurement range and resolution;
5. contact geometry, access and environmental conditions;
6. expected use frequency and severity;
7. required uncertainty or decision confidence;
8. applicable method, standard or customer requirement;
9. record and traceability needs; and
10. consequence of false acceptance or false rejection.

Equipment used only for indication may need a different control level from equipment used to accept product, set a critical process, verify a safety function or establish a reference. Identify that role explicitly. Do not allow an uncontrolled indicator to become an acceptance instrument through convenience. [S254]

Similar words describe different actions. Using them precisely prevents a certificate, workshop check or repair from being given more authority than it has. [S238]

Use the action actually performed in the record. Do not label a zero check as calibration, a calibration as repair, or an accredited certificate as product approval.

Include equipment when its result can influence product acceptance, process setting, inspection, safety, environmental monitoring, customer reporting or a reference used to check other equipment. This may include:

• calipers, micrometers, height and depth instruments;
• dial indicators, bore gauges and thickness gauges;
• surface-roughness instruments and comparison references;
• tachometers, pressure gauges, flowmeters and temperature instruments;
• electrical test instruments;
• weighing equipment and force instruments;
• timers where process duration is controlled;
• coating-thickness, gloss or colour instruments;
• master gauges, gauge blocks, reference artefacts and setting standards; and
• software or data-acquisition channels that calculate, transform or store measurement results.

Classify each item by intended role: reference, acceptance, process control, monitoring, indication only, training or out of service. Apply a clear visual and record distinction where similar-looking items have different roles.

Assign every controlled item a unique identity that remains with it through calibration, repair, storage and retirement. Where a removable accessory or interchangeable probe affects performance, control the combination or clearly record the configuration.

The register should be searchable and protected from unauthorised change. A spreadsheet can be suitable for a small system if identity, revision, access, backup and change control are maintained.

A purchase order that says only “calibrate and certify” leaves important decisions unresolved. Prepare a service specification that states, as applicable:

• equipment identity and complete configuration;
• measurement functions, ranges and units to be covered;
• calibration points or distribution through the working range;
• required method or applicable standard;
• as-found results before adjustment or repair;
• as-left results after authorised adjustment;
• required measurement uncertainty or capability;
• acceptance limits or manufacturer specification to be applied;
• whether a statement of conformity is required;
• the agreed decision rule for that statement;
• required environmental conditions;
• traceability and accreditation requirements;
• certificate content and electronic file format;
• treatment of seals, firmware, software, accessories and batteries;
• notification before adjustment, repair or work beyond the agreed scope; and
• handling, packaging and return requirements.

When the correct point, tolerance, function or accessory is instrument-specific, Refer to the product label, Technical Data Sheet, or MKTECH representative.

In Malaysia, accredited calibration laboratories operate within the Skim Akreditasi Makmal Malaysia framework. Accreditation supports confidence only for activities within the laboratory's current scope and status. Check the scope document itself. [S257; S262; S263]

Confirm that the current scope covers:

1. the measurement parameter;
2. the required range;
3. the calibration method or service type;
4. the relevant site or laboratory location;
5. the required calibration and measurement capability;
6. any field-calibration activity;
7. the equipment configuration and accessories; and
8. the accreditation status on the service date.

An accreditation symbol on a quotation, website or unrelated certificate is not enough. If a result must be covered by accreditation, ensure the requested activity appears within the current scope and that the issued certificate identifies the accredited status appropriately. For cross-border work, check the applicable accreditation body and ILAC Mutual Recognition Arrangement context through the organisation's approved supplier process. [S258]

Calibration results describe the item's condition at the time and under the stated conditions. Poor transport can change that condition.

Before dispatch:

• photograph or record the item's external condition where useful;
• confirm identity, accessories, leads, probes, standards and cases;
• clean only by the approved method;
• secure moving parts without loading precision mechanisms;
• protect measuring faces, connectors, displays and adjustment controls;
• isolate batteries or hazardous contents where transport rules require it;
• include the service specification without exposing it to oils or damage;
• use a rigid, cushioned, moisture-appropriate container; and
• apply chain-of-custody or tamper control where the risk requires it.

On return, inspect the packaging and item before release. Check for impact, corrosion, loose parts, changed seals, missing accessories, unit or software changes and abnormal operation. Do not place an instrument directly into service merely because a certificate arrived with it.

Certificate review should be performed by a person who understands both the measurement requirement and the equipment. File receipt is not technical acceptance. [S257; S260; S263]

Record who reviewed the certificate, the date, the outcome, restrictions and next action. Acceptance may be full, restricted to certain ranges or functions, conditional on another check, or rejected pending correction.

These quantities answer different questions:

• Indication is the value shown or provided by the instrument.
• Reference value is the value assigned through the calibration reference system.
• Error is the indication minus the reference value under the reported conditions.
• Correction is the value applied to compensate for an estimated systematic effect and normally has the opposite sign to the corresponding error.
• Measurement uncertainty describes the dispersion associated with the reported quantity; it is not simply the error and is not proof that the result is wrong.

Never assume the sign of a correction from an unfamiliar table. Confirm the certificate convention. Do not subtract the uncertainty from the error, use uncertainty as an automatic tolerance, or ignore it because the indicated error is small. The working measurement process may contain additional uncertainty contributions from resolution, repeatability, temperature, fixturing, reference condition, sampling, operator technique and the workpiece itself. [S238; S260; S261]

There is no universal annual interval that is correct for all equipment. Establish the initial interval from a documented assessment, then revise it as history develops. [S259]

Consider:

• consequence of an incorrect result;
• equipment type and stability;
• manufacturer's information;
• use frequency and operating hours;
• severity of vibration, shock, contamination, heat, humidity and transport;
• amount of adjustment or drift expected;
• required uncertainty and tolerance relationship;
• availability and effectiveness of intermediate checks;
• repair, overload and damage history;
• calibration provider recommendations supported by results;
• regulatory, customer or contract requirements; and
• ability to identify and reassess affected product if the item is later found outside control.

Use a conservative initial interval for unfamiliar or high-consequence applications, then extend or shorten only through an authorised, evidence-based review.

An interval is a controlled decision, not a date copied permanently from the first certificate.

Avoid changing intervals merely to reduce service cost or workload. The objective is confidence at the time of use, with efficient control supported by evidence.

Intermediate checks provide timely evidence between calibrations. Design each check around the instrument's working range, function and dominant failure modes.

A controlled check defines:

1. instrument and reference identity;
2. check point, range and function;
3. environmental and preparation conditions;
4. method and number of repetitions;
5. acceptance or warning criteria;
6. person or role authorised to perform it;
7. frequency or event trigger;
8. result and trend record; and
9. action for warning, failure or doubtful behaviour.

Suitable references may include calibrated gauge blocks, setting masters, reference artefacts, electrical sources, mass standards or stable comparison items selected through the measurement system. Protect and control the reference itself. A workshop check is only as useful as its method, reference condition, resolution and decision criteria. [S254; S256]

Checks may be triggered before use, at shift start, at defined production intervals, after transport, after suspected overload, after battery replacement, after environmental change or at the end of a measurement session. Use trends to detect gradual drift; do not wait for a result to cross the rejection limit before investigating.

Every user should be able to determine whether an item is authorised for the intended task. Status may be communicated by a durable label, electronic register, issue-control system or a combination.

Labels should not cover serial numbers, ventilation, adjustment points, displays or measuring surfaces. If a label is lost or illegible, confirm status from the controlled register before use.

Measurement equipment can change through heat, humidity, condensation, dust, abrasive particles, coolant, corrosion, magnetic fields, electrical interference, shock and poor storage.

Apply controls suitable to the equipment:

• stabilise the instrument, workpiece and reference where temperature matters;
• keep precision faces and reference artefacts clean, protected and corrosion free;
• separate instruments from grinding dust, welding spatter, chemicals and vibration;
• use cases, cabinets and transport fixtures that do not load sensitive mechanisms;
• avoid leaving equipment on hot machines, vehicles or exposed benches;
• manage batteries, leads, probes, chargers and software configurations;
• prevent unauthorised adjustment and protect calibration seals where used;
• inspect after transport or environmental excursion; and
• record relevant conditions when they affect interpretation.

Environmental control at the calibration laboratory does not remove the need to control the workshop measurement process.

Stop acceptance use when an item is overdue, fails a check, is dropped, overloaded, damaged, repaired, adjusted without authorisation, exposed to a severe environment, produces unstable results or otherwise loses its known status.

Use this response sequence:

1. stop use and identify the instrument;
2. physically and electronically quarantine it;
3. preserve the as-found condition and available evidence;
4. record the event, date, user, location and suspected cause;
5. verify or calibrate the relevant range and function without premature adjustment;
6. determine the magnitude, direction and range of any deviation;
7. identify the last known acceptable point;
8. trace measurements, batches, jobs and product decisions made since that point;
9. assess whether the deviation could change conformity or process-control decisions;
10. contain, remeasure, reinspect or otherwise disposition affected work through authorised quality personnel;
11. repair, adjust, restrict, replace or retire the equipment; and
12. document closure and any interval, training or process change.

Do not assume all product is defective merely because an instrument failed. Equally, do not assume earlier product is acceptable because the deviation appears small. Evaluate the direction of error, actual measured values, specification margins, uncertainty, range of use, check history and process evidence.

A measurement result is not the same as a conformity statement. A laboratory may report values without declaring pass or fail, or it may provide a conformity statement using an agreed decision rule. The rule describes how measurement uncertainty is considered when deciding conformity. [S257; S264]

Before requesting or using a statement of conformity, define:

• the specification and acceptance limits;
• the measurand and units;
• whether uncertainty is considered and how;
• the consequence of false acceptance and false rejection;
• the decision rule agreed with the laboratory or customer;
• any guard band or acceptance zone;
• the reporting language for indeterminate or conditional cases; and
• who retains product-release authority.

Do not create an unexplained internal “pass” by comparing only the displayed calibration error with a tolerance. Review the reported uncertainty, coverage, decision rule and intended measurement process. Product acceptance remains a separate decision under the applicable inspection plan.

Records should allow a competent reviewer to reconstruct what the item was, how it was controlled, what was found and why the next decision was made.

Retain, as applicable:

• equipment identity, owner, location, role and configuration;
• purchase, receipt and initial-acceptance information;
• calibration service specification and purchase order;
• quotations and current provider-scope evidence;
• complete certificates and controlled amendments;
• certificate-review outcome, restrictions and authorisation;
• intermediate-check results and trends;
• maintenance, repair, adjustment, damage and transport events;
• interval calculations, reviews and approvals;
• overdue or failed-equipment investigations;
• affected-product trace and disposition;
• software, firmware, probe and accessory changes;
• status changes, issue history and return-to-service evidence; and
• retirement, scrapping or transfer record.

Protect records against loss, unauthorised editing and mistaken replacement. Preserve original issued certificates and link corrections or amendments rather than silently overwriting earlier evidence.

Before releasing equipment to acceptance use, confirm:

• the physical item, serial number, asset ID and configuration match the record;
• the intended measurement process, range and function are defined;
• the calibration provider's current scope covers the required activity;
• the certificate is complete, authorised and technically reviewed;
• required points, range, functions and accessories were covered;
• as-found and as-left conditions are understood;
• results, units, error or correction sign and uncertainty are clear;
• traceability information is adequate for the intended use;
• any conformity statement uses the agreed specification and decision rule;
• the equipment meets the defined acceptance requirement;
• restrictions are visible in the register and at the point of issue;
• calibration and intermediate-check intervals have a recorded basis;
• returned equipment passed condition and functional checks;
• due date and status identification are correct;
• storage, transport and environmental controls are available;
• overdue, damaged or doubtful conditions trigger immediate quarantine;
• records can trace product measured since the last known acceptable point; and
• release is authorised by the responsible role.

The Inspection Photography section begins on the following page of the printed handbook (page 387), outside this chapter extract.