When discussing or preparing for wet calibration of flow meters, are you asking the most appropriate questions? In the past, people have asked, “Is your calibration rig traceable to NIST standards?” Today, with good reason, people are asking, “Is your calibration process accredited to ISO 17025?”

ISO 17025 is an internationally accepted standard covering “General Requirements for the Competence of Testing and Calibration Laboratories.” It includes portions of the ISO 9001 2000 standards relating to organization and management, but has details specifically focused towards calibration. ISO 17025 contains all of the requirements that calibration facilities have to meet if they wish to demonstrate that they operate a quality system, are technically competent, and are able to generate technically valid results.

Why are customers asking for calibrations to be performed in facilities with ISO 17025 accreditation instead of NIST traceability? ISO 17025 has many requirements in addition to one stating the facility must by traceable to international standards. A few of the ISO 17025 requirements are discussed here.


ISO 17025 requires that the traceable system components of the calibration process are traceable to the International System of Units (SI). The SI is the primary international standard that is referred to by organizations responsible for maintaining the national standards of individual countries. In the United States, NIST (National Institute of Standards and Technologies) is the organization responsible for maintaining national standards. NIST and similar standards organizations in participating countries (Switzerland, Germany, France, England, etc.) all refer to the same primary standards, which is the SI.

Measurement Uncertainty

To meet the requirements of ISO 17025, the measurement uncertainty (accuracy and repeatability) of the calibration rig must be defined and documented. The total uncertainty of the calibration rig is based on the uncertainty of each component in the measurement process and is a means of comparing ISO 17025 accredited rigs to each other.

There are many elements within typical flow calibration system, such as a water diverter, that are non-traceable but can significantly influence the measurement uncertainty of the calibration rig. If the movement of the water diverter is unrepeatable, the calibration facility could produce inaccurate results and have a large undocumented measurement uncertainty while still claiming component traceability to NIST. This is because NIST traceability does not refer to the measurement uncertainty of the calibration process.

Proficiency Testing

Assuring the quality of calibration results from a calibration facility is required by ISO 17025, and a commonly utilized method is inter-laboratory proficiency testing. Comparing results between different calibration facilities can either provide confidence that the calibration system is producing quality results or trigger a warning that one of the facilities may have issues that need to be investigated.

Administrative Controls and Procedures

In addition to the technical requirements, ISO 17025 defines required administrative controls and procedures to be followed by the calibration facility. Topics such as organization and management, document control, and complaint management are addressed to make sure the calibration facility functions in a manner promoting quality results.

Obtaining Accreditation

Calibration facilities are recognized as having implemented the specific requirements defined by ISO 17025 via accreditation. Obtaining accreditation involves a rigorous third party assessment to make sure that all of the requirements identified in the standard are met and periodic reviews are used to determine if the standards are maintained. There are many groups in each country that are able to perform ISO 17025 accreditation. A2LA (American Association for Laboratory Accreditation) is one of the most recognized accreditation organizations in the US for flow calibration rigs.

NIST does not govern calibration facility claims of being traceable to NIST. The responsibility is with the provider of calibration services to prepare information to support the claim of traceability, and it is the responsibility of the user of the results to assess whether the traceability claim is valid. Unfortunately, this arrangement makes it feasible for a laboratory to claim that their measurements of standards are traceable to NIST although their processes may not produce acceptable measurement results.

Calibration is an important process to assure the meter performs within the specification. Customers should choose calibration facilities that have technical competence in calibration and ISO 17025 accreditation officially demonstrates this competence. Furthermore, ISO 17025 is a standard that is recognized internationally; therefore, customers can have the same confidence in the calibration when utilizing flow meters calibrated by accredited facilities around the world (assuming similar measurement uncertainties of the calibration rigs.)