Overview
ISO 6578:2025 - "Refrigerated hydrocarbon liquids - Static measurement - Calculation procedure" specifies the validated calculation methods for converting measured volumes of refrigerated hydrocarbon liquids (LPG and LNG) under static storage conditions to standard conditions (15 °C and 101,325 kPa). The standard covers conversion to equivalent liquid or vapour volume at standard conditions, and to mass and energy (gross calorific content). It applies to fully refrigerated hydrocarbon liquids stored or transferred from tanks (including marine carriers) at near-atmospheric vapour pressures. It does not cover pressurized gases or predominantly ethane/ethylene mixtures or partially refrigerated liquids at substantially elevated pressures.
Key topics and technical requirements
- Volume-to-standard conversion: Procedures to convert liquid and vapour volumes measured in tanks to volumes at the standard reference condition (15 °C, 101,325 kPa).
- Mass calculation: Methods to determine liquid and total (liquid + vapour) mass from measured volumes, including corrections for vapour-phase content and vacuum/air corrections.
- Energy (calorific) content: Calculation of gross calorific value from composition and conversion of transferred quantity to energy units (MJ).
- Density from composition: Algorithms to calculate liquid density from chemical composition for LPG and LNG.
- Real vs ideal vapour bases: Guidance on using volumetric (ideal) and volumetric (real / compressible) bases, including compression (Z) factor use.
- Supporting data and annexes: Reference tables and informative annexes for molar volumes, correction factors, component calorific values, molar masses, compression factors, and worked examples.
- Normative references and alignment: Cross-references to ISO 91 (temperature and pressure volume correction factors) and alignment of component data with ISO 6976:2016 where applicable.
Applications and who uses this standard
ISO 6578:2025 is essential for organizations involved in custody transfer, inventory and energy accounting of refrigerated hydrocarbon liquids:
- Marine carrier operators and surveyors measuring LPG/LNG cargoes on board
- Terminal and tank farm operators performing static inventory calculations
- Custody transfer engineers and commercial energy auditors
- Measurement and metrology specialists in the petroleum and gas sectors
- Regulators and quality assurance professionals specifying accepted conversion procedures
Use cases include cargo transfer on LNG/LPG carriers, shore tank inventory, mass reconciliation for trade, and conversion of measured quantities to standard mass/energy for billing and reporting.
Related standards
- ISO 91:2017 - Temperature and pressure volume correction factors (petroleum measurement tables)
- ISO 6976:2016 - Calculation of calorific values and related component data (aligned component tables)
ISO 6578:2025 provides the technical backbone for accurate static measurement and conversion of refrigerated hydrocarbon liquids (LPG, LNG), enabling consistent custody transfer, safety-compliant inventory control, and reliable energy accounting.
