Overview
ISO/TS 29041:2008 - "Gas mixtures - Gravimetric preparation - Mastering correlations in composition" is a Technical Specification from ISO/TC 158 that examines how correlations (both intrinsic and introduced by data processing) affect the gravimetric preparation of calibration gas mixtures described in ISO 6142. The document provides tools and worked examples - notably the gravimetric preparation of a synthetic natural gas target composition (1.4 mol% N2, 1.8 mol% CO2, 9.4 mol% ethane, 3.4 mol% propane, 1.0 mol% n‑butane, 83.0 mol% methane) - to illustrate how to account for correlations when calculating composition and uncertainty.
Key topics and technical requirements
- Investigation of correlations: Analysis of a priori correlations and correlations introduced by data processing in gravimetric mixture preparation.
- Gravimetric procedure context: Uses the weighing steps, sequences and feasibility criteria defined in ISO 6142; mass measurements, buoyancy corrections, residual gas corrections and balance uncertainties are considered.
- Uncertainty treatment: Presents a generic approach to uncertainty calculation (Annex A), including formation and propagation of variance–covariance matrices for weighing data and composition.
- Unit conversion and computation tools: Describes conversion from mass to molar units (example computations using CONVERT) to integrate purity data (purity tables provided in µmol/mol).
- Purity data inclusion: Shows incorporation of purity tables and their uncertainties into final composition calculations while discussing likely independence of purity measurements.
- Gas property calculation: Covers subsequent calculations of gas property measurands (e.g., calorific values) and how correlations affect those results.
- Practical recommendations: Guidance on when the full correlation-aware calculatory scheme should be applied versus when the simplified approaches from ISO 6142 are sufficient.
Practical applications and users
ISO/TS 29041:2008 is aimed at professionals involved in the preparation and certification of calibration gas mixtures and anyone needing rigorous uncertainty budgets for gas composition:
- Calibration laboratories and metrology institutes preparing gravimetric calibration gas mixtures
- Gas producers and specialty gas suppliers producing synthetic natural gas reference mixtures
- Analytical laboratories performing gas analysis for environmental monitoring, combustion analysis, or process control
- Quality managers and accreditation bodies assessing uncertainty treatment and traceability in gas mixture preparation
- Developers of calibration protocols for gas analysers and GC systems
Related standards
- ISO 6142 - Gravimetric method for preparation of calibration gas mixtures (procedures, weighing steps)
- ISO 14912 - Mathematical and computational background for unit conversion and related calculations
- Annex A of ISO/TS 29041 provides the generic uncertainty algorithm used in the worked examples.
Keywords: ISO/TS 29041:2008, gravimetric preparation, gas mixtures, correlations in composition, uncertainty, variance-covariance, purity data, ISO 6142, calibration gas mixtures.
