Overview
ISO 6974-2:2012, "Natural gas - Determination of composition and associated uncertainty by gas chromatography - Part 2: Uncertainty calculations", defines the procedure to calculate the measurement uncertainty associated with each component mole fraction from a natural‑gas gas‑chromatography analysis carried out in accordance with ISO 6974‑1. It covers Steps 9 and 10 of the ISO 6974 workflow: calculation of uncertainties of processed mole fractions and the determination of expanded uncertainty for reporting.
Key topics and technical requirements
- Scope and purpose: Establishes how to quantify uncertainty for component mole fractions (direct and indirect components) determined by GC.
- Standards and methodology: Uncertainty calculations are performed in accordance with ISO/IEC Guide 98‑3 (GUM). Users must apply the equations and procedures given in ISO 6974‑2 to combine random and systematic contributions.
- Analysis types: Differentiates between Type 1 (fully calibrated) and Type 2 (performance evaluated) analyses; both require different uncertainty treatments.
- Sources of uncertainty:
- Uncertainty of certified reference mixtures
- Uncertainty of the analytical measurement (instrumental repeatability, run‑to‑run variation)
- Uncertainty from the data‑fitting/ calibration procedure (regression)
- Linearity and non‑linearity corrections (where applicable)
- Normalization methods: Provides procedures for mean normalization and run‑by‑run normalization of mole fractions and the associated uncertainty propagation.
- Special approaches and annexes:
- Methane‑by‑difference method (Annex A) - uncertainty treatment when methane is calculated by difference
- Relative response factor uncertainties (Annex B)
- Alternative uncertainty calculations when using ordinary least squares rather than generalized least squares (Annex C)
- Output: Calculation of standard uncertainties u(…) and expanded uncertainty U(…) using appropriate coverage factors.
Applications and typical users
ISO 6974-2 is intended for professionals who need robust, traceable uncertainty estimates for natural gas composition, including:
- Analytical chemists and laboratory managers performing GC analysis of natural gas
- Quality assurance and metrology teams in calibration laboratories
- Gas transmission/distribution operators and custody transfer metering engineers
- Gas suppliers, commodity traders and regulators who require uncertainty statements for calorific value and fiscal calculations
- Instrument manufacturers and system integrators designing GC-based analyzers
Practical uses include producing composition reports with uncertainty for calorific value and density calculations, custody transfer documentation, compliance testing, and instrument performance evaluation.
Related standards
- ISO 6974‑1:2012 - General guidelines and calculation of composition (required companion)
- ISO 6974 parts 3–6 - Specific GC methods for hydrocarbon and permanent gas determination
- ISO/IEC Guide 98‑3 (GUM:1995/2008) - Guide to the expression of uncertainty in measurement
Keywords: ISO 6974-2, natural gas composition, gas chromatography, uncertainty calculations, mole fraction, measurement uncertainty, GUM, methane-by-difference, mean normalization, run-by-run normalization.
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