Accreditation of Calibration Laboratories in the field of Legal Metrology

Introduction

Accreditation of calibration laboratories to the requirements of ISO/IEC 17025 [1] promotes confidence in their operation. The recognition granted allows the laboratory's customers and regulatory authorities, based on an evaluation by an independent third party, to have confidence that they operate competently and can generate valid results.

National accreditation systems operate similarly, thanks to the set of standards issued by the Committee for Conformity Assessment (CASCO) of the International Organization for Standardization (ISO). On the other hand, international cooperation plays a crucial role in the global consolidation of accreditation, particularly through the Global Accreditation Cooperation Incorporated (recently formed with the merging of ILAC and IAF). Regional accreditation bodies, aligned with the work of Global Accreditation Cooperation Incorporated, recognize the equivalence of the work of national accreditation bodies in these regions through recognition agreements for specific conformity assessment standards.

The International Organization of Legal Metrology (OIML) is a global intergovernmental organization with the primary objective of harmonizing the regulations and metrological controls applied by the national metrological services or related organizations of its Member States in the field of Legal Metrology. Among the documents published by OIML are the international Recommendations (OIML R), which serve as model regulations that establish the metrological characteristics of measuring instruments and the methods and means of controlling their conformity.

The former bodies ILAC and IAF both had Memoranda of Understanding in place with OIML [2] to work collaboratively on issues related to legal metrology and accreditation, recognizing that their activities are closely linked.

Legal Metrology

"Legal metrology" comprises all activities for which legal measurement requirements are prescribed, and includes units of measurement, requirements on the use of measuring instruments or systems, and measurement methods, as well as activities carried out by or on behalf of government authorities, to ensure an appropriate level of confidence in measurement results in the national regulatory environment. The scope of legal metrology may vary from country to country.

Legal metrology is one of the three fields of work into which metrology is currently divided by convention, the remaining two fields being scientific metrology and industrial metrology. The approach to these three fields of work is carried out as a substantive part of the National Metrology System (NMS) implemented, with its particularities, within each country. The NMS is structured by all organisations (public and private), together with the relevant policies, legal and regulatory framework, and practices needed to support and improve metrology activities carried out at the local level.

According to OIML document D 1 [3],  at the centre of a national metrology infrastructure should be an authority in government in charge of national metrology policy and coordinating the actions of other parts of government related to metrology issues.

The head of the NMS is sometimes the National Institute of Metrology (NMI); however, this function may be delegated to a unit with competence in the area.

The NMS is a vital part of the National Quality Infrastructure and must articulate its actions with the rest of the actors that make it up.

Among the activities carried out in the field of legal metrology are the initial and in-service verifications of instruments subject to legal metrological control. Among the instruments verified are those used in regulated fields such as commerce, commercial transactions, sports competitions, fuel transfer, and those used in public health. The extent to which instruments are subject to these controls is decided at the national level.

Next, we will focus on the verification of instruments subject to metrological control in health.

Mandatory verification of instruments that fall within the scope of legal metrology

In the field of public health, the universe of measurement instruments is growing every day, so one of the fundamental responsibilities of metrology is to ensure that all the principles observed in this science are met. 

• The form of expression of the results, including the uncertainty of the result of the measurement.
• Measurement methods.
• The traceability of the results to the units of measurement of the International System or to units of consensus.
• The competence of the personnel who address this field of metrology work.
• The design and manufacture of measuring instruments.
• The approval of the model.
• Initial verification and subsequent verification (during use).
• Metrological controls on proper use (metrological supervision) and compliance with applicable national regulations.

According to the international vocabulary of legal metrology [6], the verification of a measuring instrument is the conformity assessment procedure that results in the affixing of a verification mark and/or the issuance of a verification certificate.

Verification has historically had the fundamental purpose of providing confidence in the results of the measurements made with a "verified" instrument and consequently of the decisions made on them, this activity is an integral part of the metrological control activities carried out within the NMS.

Verification is carried out in accordance with documented procedures that take as a reference the international recommendations of the OIML or, failing that, the national regulations for each instrument. If the verifications are carried out by the national authority with competence in legal metrology, it establishes provisions to ensure the technical competence of the activities carried out.

On the other hand, in some countries, there may be situations that do not allow the national authority to directly assume verification activities, for example, where the metrological infrastructure is not sufficiently developed, the extension of the territory where the instruments are located is very large and the volume of instruments subject to mandatory verification is very high. In this scenario, certain activities, including verification, are delegated to other actors, other than the national authority in matters of legal metrology.

It is important to understand that even when the execution of the activity is carried out by a third party, the national legal metrology body supervises and controls the delegated activities.

For some types of instruments, only the approval of the model and the initial verification could be implemented; this is the case of those instruments that are marketed to private individuals, for example, digital blood pressure meters (sphygmomanometers). It can even be considered that the manufacturer of the measuring instrument is responsible for carrying out the initial verification of the measuring instrument in accordance with the provisions of its Quality Management System [7]. The above also includes the evaluation and authorization of the manufacturer to carry out this verification.

In some economies, even though for individuals the equipment only requires initial verification, if it is intended to use that same type of instrument in the provision of services, it must be periodically verified by the entity with competence at the national level and taken out of service in case it is found that the errors it has exceed the values of the maximum allowed error.

It is also common to find that manufacturers or their commercial representatives carry out the maintenance, adjustment and verification service of instruments that have a certain degree of complexity or specialization, for example, electrocardiographs, vital signs analysers and electroencephalographs. If it is decided to proceed in this way, the national authority in the field of legal metrology must clearly define the requirements to carry out this activity without prejudice to the parties involved and above all, without compromising the reliability of the results obtained with these instruments.

Competence in the execution of delegated activities

The authorization of a third party by the national legal metrology body may consider compliance with various administrative, legal, and technical requirements.

A Conformity Assessment Body (CAB) that carries out metrological control activities that partially or totally replace the activities usually carried out by the state must possess the technical competence in the field in question, which includes knowledge of the national and international regulations (OIML) that govern the measuring instrument, including any national regulations and their location within the National Metrology System.

To meet the requirements for technical competence, it is mandatory for the CAB to be accredited to ISO/IEC 17025, in addition to considering the guidance and interpretations given in OIML document D 30 [4] and applicable national requirements.

If the decision on the accreditation status of the CAB is established, it is advisable, whenever possible, that the Accreditation Body (AB) evaluating the competence be a signatory to the Global Accreditation Cooperation Incorporated recognition agreement for ISO/IEC 17025.

Accreditation of a CAB in the field of legal metrology

The AB will act at the time of evaluation in accordance with the requirements of the ISO/IEC 17011 standard [5], also including those complementary requirements established by the national authority with competence in matters of legal metrology.

It is essential to consider that the evaluation teams of these CABs comprise technical experts with expertise in legal metrology, specifically in the delegated activities. If needed, the OIML database of legal metrology experts can be consulted on their website.

The assessment must confirm that the CAB follows the requirements of ISO/IE 17025 and the additional rules of the AB, such as its policies on measurement uncertainty, metrological traceability of measurements, participation in proficiency tests, the level of risk in issuing statements of conformity, and more. The focus should be on verifying that the CAB meets the rules set at the national, regional, or international level for the activity that is being regulated.

Calibration of instruments used in the field of legal metrology

In most countries, the accreditation of activities related to measuring instruments used in the field of legal metrology is harmonized with international practice as described above.

Certainly, what is applicable for regulated instruments is that their metrological verification is carried out. However, in some countries, the development of legal metrology is incipient or does not cover all the instruments that must be subject to control; for example, metrological control schemes have been established for non-automatic weighing instruments, but automatic weighing instruments are not included. In other scenarios, there are no metrological controls for instruments in public health.

The existing gaps are sometimes filled by calibration laboratories working in the field of industrial or scientific metrology by implementing calibration methods to evaluate the metrological performance of instruments used in the field of legal metrology, particularly instruments related to public health.

Examples of the above are clinical thermometers (OIML R 7, OIML R114, OIML R 115), electrocardiographs (OIML R90), electroencephalographs (OIML R89), spectrophotometers used in clinical laboratories (OIML R 135), and sphygmomanometers (OIML R 148 and OIML R149).

For this type of OEC in the process of accreditation, evaluation teams must be aware that the recognition of competence has a direct impact on health professionals who will consider the results obtained with these "calibrated" instruments in their diagnoses.

It must be assumed from the fact, even if the presumption is not correct, that the general public may assume, for example, that an instrument calibrated by an accredited laboratory yields reliable results, however, if the selection of the standard is not adequate, if the conformity of the equipment is not evaluated (comparison of the determined metrological characteristics with respect to the defined metrological characteristics),  If an adequate decision rule is not used to assess conformity, this assumption is not correct and consequently decisions made based on measurement results may have consequences on the patient's health.

In view of the above, it is essential that, within the framework of the evaluation, it is reviewed that the method documented by the laboratory complies with the legal provisions issued in this regard by the body with national competence and, if it does not exist, with the corresponding international recommendation of the OIML or another regional or international standard.

Among the elements to be evaluated are the following:

1. Correspondence of the tests carried out with what is required for the initial or in-service verification of the measuring instrument in national or international documents. If any modification is made, the evidence of validation demonstrating that the change does not adversely affect the original performance of the method in question.
2. Participation in proficiency tests, with satisfactory results, in accordance with the scheme of the ISO/IEC 17043 standard [8] for the instruments or family of instruments included in its scope.
3. Availability of standard and auxiliary measuring instruments as recommended nationally or internationally to carry out the tests.
4. The uncertainty values for the evaluated measurement (usually the instrument indication error) should ensure an appropriate accuracy ratio with the maximum allowable error of the instrument under test (usually if the normative reference is an OIML 1/3 publication).
5. The decision rule used to issue statements of conformity.
6. Staff knowledge of the provisions relating to legal metrology for the instrument in question.

For more information, international recommendations, memorandum of understanding and other publications on the subject can be found directly on the OIML website.

Conclusions

The accreditation of calibration laboratories for scopes related to measuring instruments in the field of legal metrology must consider compliance with complementary requirements established for this type of instrument, especially those referred to in the national documents published by the national authority of legal metrology, and, in their absence, those published by the OIML. Even though the accreditation of activities related to calibration services is becoming more and more important and the public is aware of the added value of using such accredited services, evaluation teams must pay special care and consequently carry out thorough review and preparation activities when they are responsible for evaluating scopes that include instruments that are used in the field of legal metrology.

Without underestimating the rest of the calibration methods for which accreditation is granted, it is vital that the national metrology and accreditation bodies jointly draw up a particular strategy for the accreditation schemes that include these scopes, having as a common goal to ensure the competence of the accredited and the public guarantee of the decisions that are made based on the results of the measurements that are carried out with these instruments. 

References

[1] ISO/IEC 17025:2017 General requirements for the competence of testing and calibration laboratories

[2] Memoranda of Understanding (MOU) between the International Laboratory Accreditation Cooperation (ILAC) and the International Organization of Legal Metrology (OIML), and the International Accreditation Forum (IAF) and the OIML; see www.oiml.org 

[3] OIML D 1:2020 National Metrology Systems – Development of the institutional and legislative framework

[4] OIML D 30:2020 Guide for the application of ISO/IEC 17025 to the assessment of Testing Laboratories involved in legal metrology

[5] ISO/IEC 17011:2017 Conformity assessment – Requirements for accreditation bodies accrediting conformity assessment bodies

[6] OIML V 1:2022 International vocabulary of terms in legal metrology (VIML).

[7] OIML D 27:2001 Initial verification of measuring instruments utilizing the manufacturer's quality management system

[8] ISO/IEC 17043:2023 Conformity assessment – General requirements for the competence of proficiency testing providers.