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ASTM D7777 - Measuring the density, relative density, or API gravity of liquid petroleum using a portable digital density meter in field applications

What is crude oil? What is liquid petroleum?

Crude oil is a naturally occurring, unrefined petroleum product composed of hydrocarbon deposits and other organic materials. It is typically extracted from its underground location by means of oil drilling. Once extracted, crude oil is distilled using fractional distillation to produce various petroleum products. For the purposes of this article, the term “liquid petroleum” includes crude oil and liquid products made after refining crude oil, i.e. gasoline, diesel, and kerosene.

What is API gravity?

A common term to describe petroleum products is called API gravity, equivalent to the density of the product. If a type of liquid petroleum has an API gravity value greater than 10, it is lighter than water; if its API gravity is less than 10 it is heavier than water. The term “API gravity” was defined by the American Petroleum Institute (API), which is a trade association for the oil and natural gas industry in the United States.  

Why measure the density, relative density, or API gravity of liquid petroleum in the field?

Knowing the API gravity value is essential when trading petroleum goods as often the volume of a container (e.g. tanker) is known but the mass – which is the basis for account settlement – cannot be measured. Thus, the density or API gravity is used to convert volume to mass or the other way around.

The API gravity of crude oil often needs to be assessed at fuel depots, deep well drilling plants, or at pipelines. The API gravity of other fractions needs to be evaluated whenever trading occurs or before and after transport.

Accepted instruments for measuring the density, relative density, and API gravity of liquid petroleum include hydrometers, pycnometers, and digital density meters.

Which ASTM standards cover measurement of petroleum products in the field?

ASTM standard

ASTM title

Specifications

Pros of device for field use

Cons of device for field use

ASTM D7777[1]

Standard Test Method for Density, Relative Density, or API Gravity of Liquid Petroleum by Portable Digital Density Meter

This method covers the determination of the density, relative density, or API gravity of liquid petroleum products using portable digital density meters at test temperatures between 0 °C and 40 °C (32 °F to 104 °F).

  • Approx. 1 second for measurement
  •  Easy filling with built-in pump
  • Only 1 mL sample required
  • Automatic calculation of relative density and API gravity due to integrated conversion tables
  • High accuracy
  • Withstands spills and knocks
  • Ex-proof devices available
 

·         Higher one-off cost compared to pycnometers and hydrometers

ASTM D1298[2]

Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method

This method covers the laboratory determination using a glass hydrometer in conjunction with a series of calculations, of the density, relative density, or API gravity of crude petroleum, petroleum products, or mixtures of petroleum and non-petroleum products normally handled as liquids, and having a Reid vapor pressure of 101.325 kPa (14.696 psi) or less.

Values are determined at existing temperatures and corrected to 15 °C or 60 °F by means of a series of calculations and international standard tables.

 
  • Relatively inexpensive
  • Relatively good accuracy if used by skilled operator
 
 
  • Breaks easily
  • Time and effort required to obtain accurate measurement results
  • Requires skilled operator
  • Careful calibration required
  • Up to 300 mL or even  500 mL of sample required
  • Manual calculations to obtain result  using tables
 

ASTM D1217[3]

Standard Test Method for Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer

This test method covers the measurement of the density of pure hydrocarbons or petroleum distillates boiling between 90 °C and 110 °C that can be handled in a normal fashion as a liquid at the specified test temperatures of 20 °C and 25 °C. It provides a calculation procedure for the conversion of density to relative density (specific gravity).

Pycnometer pros

  • Relatively inexpensive
  • Relatively good accuracy if used by skilled operator

 

Pycnometer cons

  • Time and effort required to obtain accurate measurement results
  • Difficult and time-consuming sample preparation
  • Difficult and time-consuming cleaning
  • Requires skilled operator
  • Manual calculations to obtain result for API numbers or relative density
  • Relatively large sample required
  • 60 to 100 mL of cleaning solvents required

 

[1] www.astm.org/Standards/D7777.htm

[2] www.astm.org/Standards/D1298.htm

[3] www.astm.org/Standards/D1217

Requirements for portable digital density meters acc. to ASTM D7777 used in the field

Digital density meters used in field applications to determine the density, relative density, or API gravity of liquid petroleum products should have some or all of the following features:

  • Ex-proof certification for use in hazardous environments
  • Housing and cell which are resistant to petroleum products and solvents (protection class IP54)
  • Automatic viscosity correction
  • Automatic temperature compensation

Useful additional features are:

  • Small sample volume needed (2 mL)
  • Robust design to withstand field applications (spills, knocks, vibrations)
  • Gesture control for one-handed use to increase safety when measuring from hard-to-reach containers
  • RFID capability for quick identification at the sampling point
  • Wireless transfer of results via Bluetooth

Common units

The accepted units of measure for density according to ASTM are kilograms per cubic meter (SI unit) or grams per cubic centimeter.

Portable digital density meters measure the density and temperature of the filled-in sample at the sample temperature. The measured density and temperature are then automatically converted into density, relative density and API gravity at 15 °C, 20 °C or 60 °F.