Overview
EN ISO 8973:1999 (ISO 8973:1997) specifies a simplified, composition‑based calculation method for determining the liquid density and vapour pressure of liquefied petroleum gas (LPG). The method uses LPG compositional data (molar fractions obtained by gas chromatography) together with empirical density and vapour‑pressure factors for individual hydrocarbon components (provided in Annex A). It is intended for product quality specification and laboratory reporting - not for custody transfer quantity measurement.
Keywords: EN ISO 8973, ISO 8973, liquefied petroleum gas, LPG density calculation, vapour pressure, gas chromatography, product quality specification.
Key topics and requirements
- Scope and purpose
- Simplified calculation method based on composition and component factors.
- Not suitable for custody transfer measurement (see ISO 6578).
- Input data
- Molar composition determined by gas chromatography in accordance with ISO 7941.
- Use relative molecular masses and the density factors (kg/m3 at 15 °C) and vapour pressure factors (kPa at 37.8 °C, 40 °C, 50 °C or 70 °C) listed in Annex A.
- Calculation steps
- Convert mole fractions to mass fractions using relative molecular masses.
- Calculate liquid density at 15 °C as a weighted combination of component density factors.
- Calculate component partial vapour pressures as mole fraction × component vapour‑pressure factor and sum to obtain the mixture absolute vapour pressure.
- Convert to gauge vapour pressure by subtracting local atmospheric pressure (101.325 kPa).
- Reporting and precision
- Report density to the nearest 0.1 kg/m3 and vapour pressure to the nearest 1 kPa.
- Precision depends on gas chromatography accuracy and the empirical factor values.
- Annexes
- Annex A (normative): table of factors for common LPG components.
- Annex B (informative): bibliography and related measurement methods.
Applications and who uses it
- Quality control laboratories analyzing LPG batches for product specifications.
- LPG producers and distributors when defining or verifying product quality parameters.
- Standards committees and spec writers using a standardized calculation for contractual or regulatory specifications.
- Engineers and safety assessors estimating vapour pressures for storage, handling, venting and safety assessments (note: for custody transfer and custody‑related quantity measurement use ISO 6578).
- Useful for integration into laboratory information systems, reporting templates, and technical datasheets where a consistent, reproducible calculation method is required.
Related standards
- ISO 7941 - Analysis by gas chromatography for commercial propane and butane (molar composition).
- ISO 6578 - Refrigerated hydrocarbon liquids: static measurement (custody transfer procedures).
- ISO 3993, ISO 4256 - Methods referenced for density and vapour pressure determination (background bibliography).
