ISO 5168:2005

Overview

ISO 5168:2005 - Measurement of fluid flow: Procedures for the evaluation of uncertainties provides a standardized framework for quantifying the uncertainty in measured fluid flow-rate or flow quantity. It establishes general principles and a clear, step‑by‑step procedure (Annex A) for calculating combined standard uncertainty and expanded uncertainty, helping engineers and metrologists report flow measurements with a defined confidence level.

Key topics and requirements

• Uncertainty concepts and terminology: adopts terms from GUM and VIM (e.g., standard uncertainty u(x), combined standard uncertainty uc(y), expanded uncertainty U and coverage factor k).
• Type A and Type B evaluations: statistical (Type A) and non‑statistical (Type B) methods to assess individual uncertainty components.
• Probability distributions: guidance on rectangular, normal, triangular and bimodal distributions for Type B evaluations (Annex B).
• Sensitivity coefficients: analytical and numerical methods to determine how input quantity variations affect the measurand.
• Combination of uncertainties: rules to combine variances and covariances to obtain the combined uncertainty.
• Expression of results: computing expanded uncertainty (U = k·uc) and preparing an uncertainty budget for transparent reporting.
• Special topics and annexes: calibration of flow meters (Annex H), correlated input variables (Annex F), practical examples (Annex G), coverage factors (Annex C), and recommended procedures for multiple meters in parallel (Annex J).
• Reference to GUM: advises using the ISO Guide to the expression of uncertainty in measurement for additional depth.

Applications and who uses it

ISO 5168:2005 is used by:

• Flow metering and calibration laboratories for documenting meter performance and uncertainty on calibration rigs.
• Process and instrumentation engineers who need to quantify and report measurement uncertainty in flow‑rate results.
• Quality and compliance teams in water utilities, chemical, oil & gas, HVAC and manufacturing sectors to support measurement traceability and decision thresholds.
• Research and development groups evaluating new flow meter designs or experimental flow measurements.

Practical uses include preparing an uncertainty budget, selecting appropriate coverage factors (typically k ≈ 2–3 for common confidence levels), performing Type A/B analyses, and documenting uncertainty for reporting or contractual purposes.

Related standards

• ISO Guide to the expression of uncertainty in measurement (GUM, 1995)
• International vocabulary of basic and general terms in metrology (VIM)
• ISO 9300 (measurement of gas flow by critical flow Venturi nozzles)

Keywords: ISO 5168:2005, measurement of fluid flow, flow-rate uncertainty, uncertainty evaluation, combined standard uncertainty, expanded uncertainty, Type A, Type B, uncertainty budget, flow meter calibration.