ISO 19970:2025

Overview - ISO 19970:2025 (BOG metering on LNG carriers)

ISO 19970:2025 specifies minimum requirements for measuring and quantifying boil-off gas (BOG) used as fuel on LNG carriers, particularly during cargo transfer operations. The standard defines the BOG measurement system, methods for metering flow, procedures for calorific value (heating value) calculations, and performance, calibration and uncertainty requirements needed when the energy consumed on board is accounted in custody transfer or energy balance calculations. With some modifications, the standard also applies to BOG consumption at sea. Note: safety provisions are not covered by this document.

Key topics and technical requirements

• BOG measurement system constituents: flowmeter, associated measuring instruments (pressure, temperature, composition), flow computer and indicating device.
• Flowmeter selection and installation: any measurement principle that meets the uncertainty requirements may be used; commonly referenced meter types include differential pressure (ISO 5167-1), vortex (ISO 12764), ultrasonic (ISO 17089-1), turbine (ISO 9951), Coriolis (ISO 10790) and thermal (ISO 14511). Flowmeters must be gastight, product-compatible and suitable for gas-dangerous zones when applicable.
• Uncertainty and performance: the standard sets uncertainty requirements for BOG measurement and provides guidance and examples (Annex C) for uncertainty calculations.
• Calibration and verification: requirements for factory/laboratory calibration before installation, on-site calibration/verification after installation and periodic uncertainty verification of the BOG measurement system.
• Metering and calculation: procedures for recording flow, deriving mass/volume as required, and calculating calorific value for energy accounting (Annex B includes typical energy transfer calculations).
• Documentation and installation guidance: typical fuel gas flowmeter arrangements and instrument specifications are illustrated (Annex A).

Practical applications and users

Who benefits from ISO 19970:2025:

• LNG carrier operators and shipowners quantifying on-board BOG consumption during cargo operations
• Terminal operators and cargo surveyors involved in custody transfer and energy accounting
• Meter and system manufacturers, shipboard instrumentation engineers and integrators designing BOG metering systems
• Classification societies, maritime regulators and third‑party verifiers assessing compliance with measurement and calibration requirements
• Metrology laboratories performing flowmeter calibration and uncertainty evaluations

Practical uses:

• Ensuring accurate BOG fuel accounting during ship-to-shore transfers
• Supporting custody transfer energy adjustments and commercial settlement
• Designing compliant metering systems with traceable calibration and documented uncertainty

Related standards

• ISO/IEC Guide 98-3 (GUM) - uncertainty of measurement
• ISO 5167-1, ISO 12764, ISO 17089-1, ISO 9951, ISO 10790, ISO 14511 - flowmeter standards
• ISO 18132-1 - custody transfer measurement system terminology

Keywords: ISO 19970:2025, boil-off gas (BOG), LNG carriers, BOG metering, flowmeter, calorific value calculations, uncertainty, calibration, custody transfer measurement.