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.
