Overview
ISO 6974-1:2012 - Natural gas: Determination of composition and associated uncertainty by gas chromatography - Part 1 provides general guidelines and data‑processing procedures for converting gas chromatographic results into component mole fractions and for preparing those results for uncertainty evaluation. Part 1 defines the structure of the analysis, how to handle directly and indirectly measured components, and the conventional and alternative approaches for normalization of results. It is intended to be used together with gas chromatographic method parts (ISO 6974‑3 onward) and with ISO 6974‑2 for uncertainty calculations.
Key topics and technical requirements
- Scope and purpose: Procedures for calculating raw and processed (e.g., normalized) mole fractions from single or replicate GC analyses; supports single and multiple operation methods with multi‑point or single‑point (after performance evaluation) calibration.
- Analytical workflow: Eight-step analytical procedure covering definition of working ranges, analytical requirements, equipment selection, response characterization, relative response factors, routine calibration/QC, sample analysis, and calculation of mole fractions.
- Measurement concepts: Definitions of response, reference component, relative response factor, standard/combined/expanded uncertainty, and certified reference gas mixture (CRM).
- Normalization methods: Conventional normalization and alternatives - generalized least squares (GLS) (Annex B) for improved uncertainty assessment, and methane‑by‑difference approach (Annex C).
- Calibration and correction: Use of CRMs for calibration, response factor determination (Annex D), and pressure correction during calibration/sample analysis (Annex F).
- Quality control: Guidance on control charts and outlier testing (Annexes H and E).
- Data handling: Instructions for direct and indirect measurement, grouping of low‑level components, and procedures for processed mole fraction calculation.
Practical applications and users
ISO 6974‑1 is applicable to organizations involved in natural gas composition measurement and reporting, including:
- Analytical and metrology laboratories performing GC analysis of natural gas
- Instrument manufacturers and system integrators for on‑line and laboratory GC systems
- Calibration gas producers and QA personnel using CRMs
- Engineers and scientists calculating calorific value and other derived physical properties using composition data
Typical applications include generating composition data and associated uncertainty for calorific value calculations, calibration of gas mixtures, and routine quality assurance of GC‑based natural gas analysis.
Related standards
- ISO 6974 series (Parts 2–6) - uncertainty calculations and specific GC methods
- ISO/IEC Guide 98‑3 (GUM) - expression of uncertainty in measurement
- ISO 6143 - comparison methods for calibration gas mixtures
- ISO 10723 - performance evaluation for on‑line analytical systems
- ISO Guide 35 - reference materials (CRMs)
Keywords: ISO 6974‑1:2012, natural gas composition, gas chromatography, mole fractions, uncertainty, calibration, relative response factor, normalization, certified reference gas mixture.
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