ISO 11631:1998

Overview

ISO 11631:1998 - Measurement of fluid flow: Methods of specifying flowmeter performance provides a standardized framework for how manufacturers and specifiers should describe the metrological performance of flowmeters. It applies to flowmeters used in closed conduits and open channels, and defines the terms, quantitative methods and presentation formats needed so users can predict performance under specified conditions. The standard also mandates how to state traceability, uncertainty of measurement, and conditions of use.

Key topics and requirements

• Terms and definitions: Standardizes vocabulary for flowmeter characteristics (e.g., bias, repeatability, hysteresis, resolution, K‑factor, rangeability, turndown, span). This ensures consistent technical specifications and comparability between products.
• General requirements: Specifications must describe the complete flowmeter package, identify model/order numbers, and state compliance with relevant international standards.
• Traceability: Requires a clear statement of how measurement results relate to national or international standards through an unbroken chain of comparisons, with stated uncertainties.
• Uncertainty of measurement: Calls for estimating and reporting measurement uncertainty in accordance with ISO/TR 5168 and related guidance.
• Conditions of use: Performance must be quoted for specified environmental and operational conditions (e.g., fluid pressure, conduit configuration, impurities).
• Traceability groups: Provides classification of flowmeters by traceability group to help users assess expected measurement quality.
• Linearity and conformity: Defines conformity concepts (independent, terminal‑based, zero‑based) and linearity; Annex A illustrates linearity cases.
• Repeatability: Specifies how to determine repeatability (Annex B is normative and gives the calculation method).
• Calibration and performance reporting: Defines calibration scope and the form of calibration results (factors, curves, certificates), distinguishing calibration from adjustment.

Applications and who uses it

ISO 11631 is practical for:

• Flowmeter manufacturers - to prepare clear, comparable product specifications and claims.
• Calibration and test laboratories - to report calibration results, uncertainties, and repeatability consistently.
• Procurement and asset owners (utilities, chemical, oil & gas, water management) - to evaluate vendor performance claims and select meters suited to operational conditions.
• Metrology and QA professionals - to ensure traceability, document uncertainty budgets, and verify compliance.
• System integrators and process engineers - to predict in‑service performance given specified installation and environmental conditions.

Related standards

• ISO/TR 5168 - referenced for estimating uncertainty of a flowrate measurement.
• ISO 7066‑1 and ISO 7066‑2 are cited for least‑squares methods related to conformity/linearity.

Using ISO 11631 helps ensure transparent, comparable, and traceable flowmeter performance data - critical keywords for "flowmeter performance", "measurement of fluid flow", "traceability", and "measurement uncertainty".