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 U(…) using appropriate coverage factors.
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