ASTM International (formerly American Society for Testing and Materials, as of 2001 “ASTM International”) defines the test method used for determining the density, relative density, and API Gravity of crude oils in ASTM D5002. The procedure defined in this standard applies to crude oils that can be handled in a normal fashion as liquids at test temperatures between 15 °C and 35 °C utilizing either manual or automated sample injection equipment.
ASTM D5002 – Standard Test Method for Density, Relative Density, and API Gravity of Crude Oils by Digital Density Analyzer
What are the exact requirements for density measurement according to ASTM D5002?
When measuring crude oils with high vapor pressures, ASTM D5002 stipulates that “appropriate precautions are taken to prevent vapor loss during transfer of the sample to the density analyzer”. This is especially true when handling lighter crude oil.
These precautions include:
- The use of sealed containers filled to the top (no headspace)
- Keeping samples at low temperatures
Care must be taken to avoid bubbles in the filled-in sample. With heavier crude oils, in particular, it is useful to have a density device with a temperature cell operated at elevated temperatures to keep the crude oil in a liquid state for measurement and minimize the risk of bubbles. Filling crude oil requires a heated syringe or a single sample changer with a heated syringe or a sample changing magazine which heats the sample and tubes.
Why measure density, relative density, or API Gravity of crude oils?
The density values of crude oil are required:
- to determine the quality
- when converting the measured volume of crude into the volume at the standard temperatures (15 °C, 60 °F)
- when converting the measured mass of crude into volume
- for fiscal or custody transfer accounting calculations (in this case the measured water and sediment values are often required for the calculations)
The accepted units of measurement for density according to ASTM are kilograms per cubic meter (kg/m3, SI unit) or grams per milliliter (g/mL).
Alternative ASTM standards for density measurement
Non-digital methods for density measurement (pycnometers, hydrometers) are described in the following ASTM standards:
D287 Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method)
D941 Test Method for Density and Relative Density (Specific Gravity) of Liquids by Lipkin Bicapillary Pycnometer
D1217 Test Method for Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer
D1298 Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method
Other relevant ASTM standards for digital density measurement
D7777 Measuring the Density, Relative Density, or API Gravity of Liquid Petroleum using a Portable Digital Density Meter in field applications. This standard describes the use of portable density meters (also called digital hydrometers) at a required precision of 1 kg/m³.
D4052 Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter. This standard covers fuel density measurements by means of digital density meters in the laboratory at a required precision of 0.1 kg/m³; it further describes how measuring errors such as gas bubbles in the liquid can be detected by means of physical or optical methods.