Overview
EN ISO 3993:1995 (ISO 3993:1984) specifies the pressure hydrometer method for determining the density or relative density of liquefied petroleum gas (LPG) and light hydrocarbons. The procedure covers sampling, apparatus, calibration and measurement under controlled temperature. It is intended for laboratory and field density testing but notes that only measurements made with a certified hydrometer are suitable for custody transfer or fiscal calculations.
Key topics and requirements
- Scope & pressure limit: Apparatus must not be used for materials with gauge vapour pressures higher than 1.4 MPa (absolute 1.5 MPa) at test temperature.
- Measurement principle: Purge system, fill a pressure hydrometer cylinder so the hydrometer floats freely, record hydrometer reading and sample temperature.
- Temperature control: Use a water bath to maintain the cylinder at 15 ±0.2 °C, 20 ±0.2 °C, or 60 ±0.5 °F (reference temperatures commonly used).
- Calibration: Either use a certified hydrometer (required for fiscal/custody transfer) or calibrate hydrometers against certified reference liquids (propane, butane). Apply calibration corrections when scale errors exceed specified limits.
- Apparatus & safety: Glass or transparent-plastic hydrometer cylinder with needle valves and pressure gauge; thermometers of sufficient sensitivity; protective shields and strict cleaning rules (avoid ketones, alcohols, aromatics that attack plastics). Vent cylinder to atmospheric pressure after each test.
- Precision: Stated repeatability and reproducibility limits (method precision derived from interlaboratory studies).
- Annex: Provision for thermohydrometers is included.
Practical applications
- Quality control and routine density measurement of LPG and light hydrocarbons in production, storage and distribution.
- Commercial and regulatory testing where accurate density/relative density is needed for inventory, blending and product specification.
- Custody transfer / fiscal measurement when using a certified hydrometer to determine mass/volume relationships for trade.
- Suitable for testing laboratories, refineries, LPG distributors, inspection bodies and metrology services performing LPG density verification.
Related standards
- ISO references in the standard include methods and tables for temperature corrections and alternate calculation approaches (e.g., ISO 6578-gas chromatography alternative, ISO 5024, and petroleum measurement tables referenced in ISO 911). Industry tables (ASTM D1250, API) are also cited for corrections and conversions.
Keywords: EN ISO 3993, ISO 3993, LPG density determination, pressure hydrometer method, liquefied petroleum gas, hydrometer calibration, custody transfer, fuel metrology.
