Overview
ISO 6974-3:2018 - "Natural gas - Determination of composition and associated uncertainty by gas chromatography - Part 3: Precision and bias" defines the expected precision and gives guidance on bias assessment for gas chromatographic analysis of natural gas when performed in accordance with ISO 6974-1. Based on statistical evaluation of 14 proficiency testing (PT) exercises (2012–2014), the standard provides repeatability and reproducibility estimates for methane and non‑methane components and describes how laboratories should compare their internal performance to these reference values.
Key topics
- Precision definitions: clear definitions for repeatability, reproducibility, measurement precision, random and systematic error, and laboratory bias (aligned with ISO/IEC/JCGM terminology).
- Reference precision values:
- Methane (normalized results): repeatability s_r = 0.038% (relative); reproducibility s_R = 0.09% (relative).
- Non‑methane components: precision expressed by empirical relationships:
- ln(s_r) = −5.64 + 0.58 × ln(x)
- ln(s_R) = −4.28 + 0.715 × ln(x)
(where x is the mole fraction)
- Typical example values for non‑methane components provided for mole fractions 0.01, 0.1, 1 and 10%.
- Statistical practice and tests:
- Use of normalized mole fraction data for comparisons.
- Recommended number of repeat measurements: 10 (minimum 5 if necessary).
- Checks for normal distribution (e.g., Anderson–Darling) before reproducibility analysis.
- Use of chi‑squared tests to compare site precision with reference reproducibility.
- Bias assessment:
- Bias may be estimated by comparing average measurements to a certified reference material or assigned PT value.
- Frequent participation in PT and use of certified reference materials are recommended for bias control.
Applications
ISO 6974-3:2018 is practical for:
- Analytical laboratories performing natural gas and LNG composition analysis to validate method performance and confirm competence.
- Quality assurance managers implementing Statistical Quality Control (SQC) and working measurement standard programs.
- Acceptance testing of newly installed gas chromatographs and routine performance checks.
- Proficiency test providers and accreditation bodies (ISO/IEC 17025) assessing laboratory bias and reproducibility.
- Instrument manufacturers benchmarking chromatograph performance against industry reference precision.
Related standards
- ISO 6974-1 (setup and measurement method) - normative reference.
- ISO/IEC 17025 - laboratory competence.
- ISO 5725 - precision (repeatability/reproducibility) of measurement methods.
- ISO 17043 - proficiency testing guidance.
Keywords: ISO 6974-3:2018, natural gas, gas chromatography, precision and bias, repeatability, reproducibility, measurement uncertainty, proficiency testing, laboratory quality control.
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