Overview
EN ISO 6974-2:2012 (ISO 6974-2:2012) specifies how to calculate the measurement uncertainty associated with component mole fractions determined by gas chromatography in natural gas. Intended to be used together with ISO 6974-1 (general guidelines and composition calculation) and the analysis methods in ISO 6974-3 and subsequent parts, this part (Part 2) covers the formal uncertainty-calculation steps required after a chromatographic analysis has produced raw and processed mole fractions.
Key points:
- Focuses on uncertainty calculations for mole fractions of natural-gas components.
- Implements uncertainty assessment in accordance with ISO/IEC Guide 98-3 (GUM).
- Complements the analysis workflow in ISO 6974-1 by covering Steps 9 and 10 (calculation of standard and expanded uncertainties).
Key Topics and Requirements
- Scope: Defines the process to determine uncertainty for each gas component’s mole fraction following ISO 6974‑1 procedures.
- Normative references: ISO 6974-1:2012 and ISO/IEC Guide 98-3 (GUM).
- Analysis types: Distinguishes uncertainty contributions for Type 1 and Type 2 analyses (as defined in ISO 6974-1), addressing random and systematic sources.
- Uncertainty sources: Includes uncertainty of certified reference mixtures, analytical measurement uncertainty, and uncertainty from data fitting (regression).
- Methods and alternatives:
- Conventional normalization (noted as conservative for some methods).
- Generalized least squares (GLS) normalization (annexed in ISO 6974-1) for improved uncertainty assessment.
- Methane-by-difference approach (Annex A) and alternative calculations (Annex C).
- Treatment of relative response factors (Annex B).
- Symbols and definitions: Standardized symbols (u(), U(), x*, x, response factors, etc.) for consistent reporting.
Applications and Users
EN ISO 6974-2:2012 is used by organizations requiring robust uncertainty quantification for natural gas composition, including:
- Analytical laboratories performing gas-chromatographic composition analyses.
- Calibration and reference-gas producers.
- Gas transmission and distribution companies for custody transfer, billing, and quality reporting.
- Instrument manufacturers and validation engineers designing GC systems.
- Regulators and auditors assessing measurement traceability and conformity.
Practical applications include providing uncertainty inputs for calorific value calculations, physical-property modelling, and compliant reporting of gas composition in contractual and regulatory contexts.
Related Standards
- ISO 6974-1:2012 - General guidelines and calculation of composition (mandatory companion).
- ISO/IEC Guide 98-3 (GUM) - Guide to the expression of uncertainty in measurement.
- ISO 6974 series (Parts 3–6) - Specific GC methods and component scopes referenced for analysis procedures.
Keywords: ISO 6974-2, EN ISO 6974-2:2012, natural gas, gas chromatography, uncertainty calculations, mole fraction, GUM, calibration, methane-by-difference, generalized least squares.
