Overview
ISO 3675:1998 - "Crude petroleum and liquid petroleum products - Laboratory determination of density - Hydrometer method" - defines a laboratory procedure for measuring density at 15 °C of crude oils and liquid petroleum products using a glass hydrometer. The method applies to mobile, normally liquid materials with a Reid vapor pressure (RVP) of 100 kPa or less. It covers sample preparation, apparatus, measurement technique, corrections (meniscus, calibration temperature), and conversion to standard density units using petroleum measurement tables.
Key topics and requirements
- Scope and applicability: Designed for transparent liquids; can be adapted for viscous or opaque liquids (with temperature control or meniscus correction).
- Primary measurement: Hydrometer scale reading converted to density at 15 °C using standardized tables.
- Apparatus:
- Glass hydrometers conforming to ISO 649-1.
- Hydrometer cylinder (clear glass, plastic or metal) sized to allow correct floatation.
- Thermometer and optionally a constant-temperature bath (bath control capability ±0.25 °C is specified).
- Sample handling:
- Sampling per ISO 3170 or ISO 3171.
- Detailed guidance to minimize loss of light components (volatile and waxy materials), including closed-system mixing and temperature limits before testing.
- Procedure highlights:
- Bring sample, cylinder and hydrometer to test temperature (preferably near 15 °C).
- Read hydrometer for transparent liquids at the principal surface; read differently for opaque liquids and apply meniscus correction.
- Apply hydrometer and thermometer corrections, then convert readings using Petroleum Measurement Tables described in ISO 91-1:1992.
- Verification & calibration:
- Hydrometers should be verified against traceable references and recalibrated as necessary (recommendation: at least every five years).
- Safety: Standard includes a warning that hazardous materials and operations may be involved; users must apply appropriate safety practices.
Applications and users
ISO 3675:1998 is used in:
- Petroleum testing laboratories for density and API gravity determination.
- Refineries and distribution terminals for quality control and inventory reconciliation.
- Pipeline and terminal custody transfer where laboratory density measurement is required.
- Petrochemical and lubricant testing for product specification and blending calculations.
Typical users: laboratory technicians, quality managers, oil industry metrologists, custody-transfer auditors, and regulators.
Related standards
- ISO 91-1:1992 (Petroleum measurement tables - conversion to 15 °C)
- ISO 649-1 (Density hydrometers - specification)
- ISO 3170 / ISO 3171 (Sampling of petroleum liquids)
- IP methods (e.g., IP 389 referenced for wax appearance temperature)
Keywords: ISO 3675:1998, hydrometer method, density at 15 °C, crude petroleum, liquid petroleum products, hydrometer cylinder, meniscus correction, petroleum measurement tables.
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