FAQ on Measurement Technology and Calibration

The reliable monitoring of measuring and testing equipment is an essential prerequisite for traceable measurement results. At the same time, there is often uncertainty in practice regarding which information is binding, the significance of an accredited calibration, and what measures are required following a repair or adjustment.

The answers below are based on normative definitions, legal requirements and experience gained from metrological practice. They do not claim to be exhaustive and do not constitute legally binding information. For specific applications, the applicable laws, standards, industry requirements and internal management system guidelines must always be taken into account.

Why must measuring equipment be monitored?

Measuring equipment used to assess the quality, safety or conformity of a product or process must, as a general rule, be monitored. ISO 9001 addresses the requirements for monitoring and measurement resources in clause 7.1.5.

An organisation must ensure that the measuring and testing equipment used is suitable for its intended purpose. In addition, appropriate documented information must be retained as evidence. This includes, in particular, calibration and test certificates, as well as other records that provide verifiable documentation of the condition and suitability of a measuring instrument.

The scope of the monitoring of measuring equipment depends on the specific area of application. The decisive factor is not merely whether a device is capable of measuring, but whether the result obtained with it is sufficiently reliable and traceable for the intended application.

What is the significance of the calibration sticker on the measuring instrument?

The calibration sticker is primarily a visible reminder and source of information. It usually contains details of the last calibration, the next due date or the identification of the measuring instrument.

However, the documentation relevant for assessment is the associated calibration or test certificate. This documents the measurements carried out, the values determined, the measurement deviations, the measurement uncertainties and, where applicable, statements of conformity.

A calibration label cannot fully convey this information. It therefore replaces neither the calibration certificate nor the proper documentation required for measuring equipment management.

Is the calibration entry in the device software binding?

Many measuring instruments allow the date of the last calibration or the next due date to be recorded directly in the instrument’s software. Such an entry is essentially comparable to a calibration label: it serves as a reminder and assists the user in monitoring the instrument.

However, the entry alone does not have the binding status of a certificate. It is often not sufficiently clear who recorded or amended the information. Furthermore, such entries are not always protected against subsequent alterations.

The calibration certificate is therefore the authoritative document proving that a calibration has been carried out. It enables the calibration to be linked to the measuring instrument and documents the results and the conditions of the calibration. Only when combined with a regulated filing system and appropriate measuring instrument management does a traceable chain of documentation emerge.

What does the calibration due date mean?

The due date, often referred to as the ‘due date’, indicates the date on which the next verification of the measuring instrument is required in accordance with the specified monitoring interval.

The calibration itself always describes the condition of the measuring instrument at the time it is carried out. It is therefore no guarantee that the measuring instrument will remain within the required tolerances up to a specific future date. Rather, the due date represents an organisational requirement within the context of measuring instrument surveillance.

By the specified due date at the latest, the continued use of the measuring instrument must be assessed and the appropriate action taken. As a rule, this involves recalibration or re-inspection.

The organisation must clearly define within its management system whether a measuring instrument may still be used on the due date. Factors to be taken into account include the criticality of the measurement, internal processes, applicable standards, and any legal or customer-specific requirements. A clear policy prevents differing interpretations and reduces the risk of an overdue measuring instrument being unintentionally used further.

Is SCS calibration required, or is non-accredited calibration sufficient?

Whether an accredited SCS calibration is required depends on the field of application and the applicable requirements. In various regulated sectors, such as medical technology, the automotive industry or aviation, accredited calibrations may be expressly required or implied. The exact wording of the relevant standards, regulations and customer specifications must be carefully examined and interpreted correctly.

Even if an accredited calibration is not expressly required, the user must still assess how the technical competence of the commissioned calibration laboratory is demonstrated. The designation ‘calibration laboratory’ is not in itself sufficient proof of competence. Key factors include:

• the technical qualifications of the staff,
• suitable and documented calibration procedures,
• metrological traceability,
• the specification of measurement uncertainty,
• the suitability of the reference standards used,
• the quality of the certificates issued,
• control of environmental conditions,
• and experience with the relevant type of equipment and measurement quantity.

In the case of an SCS calibration laboratory, competence within a defined scope of accreditation is assessed and regularly monitored by the Swiss Accreditation Service. Accreditation thus instils confidence in the professional performance of the calibrations offered within this scope.

However, it does not relieve the client of the responsibility to correctly define the required scope of calibration.

Does an SCS calibration guarantee that a measuring instrument has been fully calibrated?

An SCS calibration confirms that the measurement quantities and measurement points stated in the certificate have been calibrated in accordance with the accredited procedure. However, it does not automatically guarantee that all functions, measurement ranges and characteristics of a measuring instrument are fully covered.

A modern measuring instrument may comprise various measured quantities, functions and accessory components. It is up to the user or the person responsible for the instrument to determine which of these are relevant to the intended application. The scope of calibration should therefore be clearly defined at the time the order is placed.

The following questions are helpful in this regard:

• Which measurement quantities are actually used?
• Which measurement ranges are relevant for the application?
• At which measurement points is verification required?
• Which accessories influence the measurement result?
• Are there any regulatory or customer-specific requirements?
• Is a declaration of conformity also required?

In certain specialist areas, the scope of calibration is specified in more detail by a standard. This is the case, for example, with certain EMC calibrations, where CISPR standards may define requirements for equipment characteristics, measurement procedures or calibration scopes. If a calibration is expressly commissioned and carried out in accordance with such a standard, the documented scope must comply with the relevant standard requirements.

In this case too, it is advisable to check the certificate carefully. The key factor is whether the measurement parameters and functions required for the specific intended use are actually included.

Does a measuring instrument need to be calibrated after repair?

If components relevant to quality or measurement are replaced or modified during a repair, calibration or recalibration is generally required. Only through a metrological check can it be determined and documented whether the device is functioning correctly again following the repair and meets the required specifications.

A functional test alone is not always sufficient for this purpose. It can confirm that the instrument can be switched on and operated. However, it does not necessarily provide evidence that the displayed values are correct.

A data report or calibration certificate documents the results following the repair. This provides the user with a verifiable basis for the approval and continued use of the measuring instrument.

Why should calibration be carried out before and after an adjustment or repair?

Where possible, calibration or a suitable partial calibration should be carried out both before and after any adjustment, setting or repair.

Calibration prior to the intervention documents the condition found. These so-called initial values or ‘as-found’ values show whether the measuring instrument was within the specified requirements prior to the repair or adjustment. If unacceptable deviations are detected, an assessment must be made of what impact this may have had on previous measurements.

Depending on the application and criticality, this may give rise to additional measures, for example:

• the review of previous measurement results,
• re-measurements on products or equipment,
• an assessment of affected processes,
• the withdrawal of products,
• informing customers,
• or, in particularly critical cases, a recall.

Calibration following the intervention documents the new condition. The so-called initial values or ‘as-left’ values confirm whether the measuring instrument is once again operating within the specified requirements following adjustment or repair.

Documenting both states thus enables a comprehensive assessment. Without initial values, it is true that it can be established that the instrument is operating correctly after the intervention. However, the state prior to the intervention – and thus the potential risk associated with measurements already carried out – remains unknown.

What is the difference between a test and a calibration?

During calibration, the readings or output values of a measuring instrument are compared with the values of a suitable reference standard. The results, deviations and measurement uncertainties are documented. Calibration therefore provides quantitative information about the instrument’s metrological behaviour.

An inspection, by contrast, assesses whether specified requirements are met. The result is often documented in the form of a statement such as ‘in order’, ‘not in order’, ‘pass’ or ‘fail’.

In practice, a test may be based on measured values without all individual values being listed in the test report. The scope and documentation depend on the test procedure and the underlying requirements. A test is therefore not simply equivalent to a calibration.

Put simply, the two terms can be distinguished as follows:

• Calibration:  Determination and documentation of measured values, deviations and measurement uncertainties.
• Test:  Assessment of whether defined requirements are met.
• Adjustment:  Intervening in the measuring instrument with the aim of altering or minimising its deviation.
• Repair:  Restoration of the intended function of a faulty or damaged instrument.

Which of these activities is required depends on the condition of the measuring instrument, its intended use and the applicable requirements.

Reliable measurement results require clear processes

Effective measurement equipment control does not consist solely of a calibration sticker or a recorded due date. What is crucial is the interplay between suitable measurement equipment, appropriately defined calibration intervals, professional calibrations, complete certificates and traceable documentation.

Equally important is the correct definition of the scope of calibration. Accreditation instils confidence in the competence of a calibration laboratory within its recognised scope. However, whether the calibration carried out fully covers the specific intended use must be assessed on the basis of the measurement quantities, measurement ranges, functions and normative requirements involved.

Documented calibration is of central importance, particularly following adjustments and repairs. It not only confirms the current condition of the measuring instrument but also enables a well-founded evaluation of previous measurement results, provided that input values are available.