Overview
ISO 91:2017 - Petroleum and related products - Temperature and pressure volume correction factors (petroleum measurement tables) and standard reference conditions - defines how to convert measured hydrocarbon volumes at ambient conditions to standard reference conditions for transactional and custody-transfer purposes. The standard covers temperature volume correction factors, compressibility factors for volumes measured under pressure, and the specification of standard reference conditions (see Annex B).
Keywords: ISO 91:2017, temperature volume correction factors, petroleum measurement tables, compressibility factors, standard reference conditions, custody transfer.
Key topics and technical requirements
- Purpose: Convert observed liquid hydrocarbon volumes to equivalent volumes at agreed reference temperature and pressure for accounting and trade.
- Volume correction factors: Guidance and sources for temperature and pressure correction factors (petroleum measurement tables) to adjust thermal expansion and compressibility.
- Compressibility factors: Correction of volumes measured under pressure to the equilibrium (saturation) pressure corresponding to the measured temperature.
- Commodity groups: Recognizes three primary groups - crude oil, refined products, and lubricating oils - and a special application category for experimentally derived coefficients.
- Defining limits (examples from the standard):
- Temperature: approximately −50 °C to +150 °C (−58 °F to 302 °F)
- Pressure: 0 to 1 500 psig (≈ 0 to 103.4 bar)
- Density ranges for crude and products (typical): ~611 to 1 164 kg/m3 (at standard base temps)
- Reference implementation: For custody transfer, ISO 91:2017 requires reference to API MPMS Chapter 11.1-2004 (Adjunct to ASTM D1250-04 / IP 200/04) including Addendum 1‑2007, which provides the implementation procedures and CTPL (correction factor for temperature and pressure of the liquid).
Practical applications and users
ISO 91:2017 is used wherever accurate, consistent volumetric measurement of liquid hydrocarbons is required for trade, accounting or regulatory purposes:
- Custody transfer and commercial trading of crude oil, refined fuels, NGL and LPG
- Tank gauging and inventory management at terminals and storage facilities
- Pipeline and metering systems where volume at standard conditions must be reported
- Measurement and metrology engineers, terminal operators, trading houses, fiscal metering teams, and regulators who need standardized correction methods
- Software developers and system integrators implementing measurement and conversion algorithms compliant with industry standards
Related standards
- API MPMS Chapter 11.1 (2004) / Adjunct to ASTM D1250-04 / IP 200/04 (Addendum 1-2007) - primary reference implementation
- API MPMS Chapters 11.2.x and 11.5 - compressibility and density intraconversions
- API MPMS Chapter 11.2.4 / GPA TP-27 - NGL and LPG temperature corrections
- ISO 13443 - standard reference conditions for natural gas (excluded from ISO 91:2017)
ISO 91:2017 consolidates international best practice for temperature and pressure volume correction factors, ensuring consistent and auditable volumetric conversions used across the petroleum industry.
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