[Translate to English:] Messgeräte ausserhalb der Toleranz - was tun?

Die Genauigkeit von Messgeräten ist entscheidend für die Einhaltung von Qualitätsstandards in der Fertigung und Laborumgebung. Doch was passiert, wenn ein Gerät bei der Kalibrierung ausserhalb der Toleranz liegt? Dieser Artikel beleuchtet die Anforderungen an Messgeräte, geht den Ursachen von Abweichungen auf den Grund und erläutert, welche Massnahmen Unternehmen ergreifen sollten, um die Risiken zu minimieren und zukünftige Abweichungen zu verhindern.

The calibration of measuring instruments is governed by several ISO standards, relevant depending on the field of application and industry. Key standards include ISO/IEC 17025:2017 (testing and calibration laboratories), ISO 13485:2016 (medical devices), ISO/TS 16949:2009 (automotive industry), and ISO 9001:2015 - the most widely used ISO standard globally, which defines the requirements for a quality management system and is applied across various sectors and organizations.

The standards require, with similar wording, that measuring instruments used for monitoring, measurement, or to support testing and calibrations must be suitable and calibrated or verified to ensure regulatory compliance. During calibration, the instrument’s displayed values are compared with a reference standard, the measurement deviation (measurement accuracy) is documented, measurement uncertainty is calculated, and a calibration certificate is issued. Verification, on the other hand, is a qualitative test to determine if a specific requirement is met, without providing an assessment of measurement uncertainty or deviation.

It is necessary to categorize measuring instruments into those subject to mandatory testing and those that are not. Instruments required for safety-, quality-, and process-relevant measurements are subject to mandatory testing. This also requires defining an interval, indicating how frequently calibration or verification should be performed. This interval is determined individually based on factors such as environmental influences, frequency of use, and the specific application in the process.

Some standards, particularly in highly regulated industries such as medical technology, pharmaceuticals, or precision manufacturing, require calibration using standards traceable to international or national standards. These standards or instruments must therefore be calibrated by accredited bodies, which are subordinate to the national standard, like the Federal Institute of Metrology (METAS) in Switzerland.

An accredited SCS laboratory, evaluated and accredited by the Swiss Accreditation Service (SAS), ensures metrological traceability by linking reference devices through an unbroken chain of comparative measurements to national or international standards. The technical competence in calculating measurement uncertainties and choosing the appropriate method is formally recognized in laboratories accredited to ISO/IEC 17025, and their quality management system complies with ISO/IEC 9001:2015.

Causes of Measurement Deviations

Every measuring instrument has a certain tolerance due to its design, the components used, and environmental conditions. As long as the maximum tolerance values, specified by the manufacturer in the technical data, are not exceeded, there is no cause for concern. A deviation during calibration is only detected when the measured values fall outside the permissible tolerances. Tolerance can, in this context, also be referred to as the total deviation.

There are several possible causes for such deviations. Mechanical components may wear out with constant use, while electronic components may lose accuracy over time. Extreme temperatures or frequent temperature fluctuations can significantly impact measurement accuracy. High humidity can lead to corrosion, while strong shocks or repeated vibrations can damage mechanical and electronic parts.

Improper handling, usage, and maintenance, such as rough handling, incorrect transportation or storage, or operating devices beyond their specified limits, can also lead to calibration errors. These factors may occur individually or in combination, causing an instrument to be outside the set tolerances during calibration.

Handling Calibration Deviations

To ensure the quality and safety of measurements, the aforementioned ISO standards require specific measures for handling measuring instruments that fall outside tolerance during calibration. A systematic approach to managing such instruments includes documenting the deviations. This involves careful recording of results as well as the creation of a calibration certificate. In addition, technical analyses should be conducted to identify the causes of the deviation.

A risk assessment is conducted to determine if the detected deviation is safety-relevant. This analysis examines the potential consequences of an erroneous measurement and assesses the risk that the deviation may have affected the quality of products or services.

If this analysis reveals actual uncertainty, the next step is to identify the measurement activities in which the faulty device was used. It can be assumed that the entire period between the last proper calibration and the detected deviation is suspect, and all measurements taken during this interval must be reviewed. Depending on the application, statistical samples of the produced units should be taken and tested, or a comprehensive review of all products manufactured with the faulty instrument may be necessary to ensure they meet quality requirements.

Additionally, customers who received products or services affected should be informed, particularly if these involve safety-relevant products or services. In cases of significant risk or confirmed quality defects, a product recall may be necessary.

Handling Affected Measuring Instruments

For the affected measuring instrument with a calibration certificate stating “failed” for conformity, there are three possible actions to restore the quality and safety of measurements. First, the instrument can be taken out of service. Second, it can be classified as “passed with restrictions.” Third, an adjustment of the instrument may be carried out. It should be noted that the conformity statement is not necessarily part of a calibration certificate but is often provided by laboratories to facilitate the interpretation of measured values for users.

In the case of the first option, taking the instrument out of service, it is necessary to document the “as found” calibration with a certificate. This is especially important for ensuring the traceability of past measurements since, in the case of nonconformity, the focus is primarily on the past. This documentation may be crucial to determine if and to what extent incorrect measurements have affected the quality of products or services.

In the second case, the instrument is used but with certain restrictions. The classification “passed with restrictions” is often appropriate if the instrument is not fully compliant but can still be used to an acceptable extent for specific applications. However, clear documentation and communication of these restrictions to all users are essential to avoid misunderstandings and misapplications.

In adjustment – the third option – an instrument is set to minimize a systematic measurement deviation as much as possible. Depending on the type of instrument, this may include adjusting one or more measurement points. Unlike calibration, adjustment involves an active intervention in the measurement system, requiring a follow-up calibration with the “as left” result to ensure full traceability of all measurement results and to safeguard the integrity of measurement data.

Importance of Calibration in Metrology

In metrology, one thing is certain: each measurement value and size undergoes changes – the timing of these changes, however, is variable. Therefore, regular calibration of measuring instruments, possibly supplemented by validation between calibrations, is the only way to detect deviations in measurement behavior early on. It is also advisable to consider whether future calibration intervals should be shortened to detect deviations at an early stage.

When defining new calibration intervals, various factors must be considered, such as environmental influences, frequency of use, and the specific application of the instrument in the process. Based on previously measured values, it is possible to estimate when one or more measurement points may no longer meet the required accuracy. On this basis, realistic decisions can be made regarding appropriate intervals to minimize future risks.

Moreover, it is advisable to select calibration laboratories that issue a conformity statement. This conformity statement, derived from the calibration results, is presented in a clear and understandable form. This facilitates the interpretation of results for the user and often makes it possible in the first place.