Measuring the density of fuels when trading and transporting

This video explains the use of density measurement when converting mass to volume for fuel trading purposes.

Fuel density is an important quality parameter of diesel, gasoline, and jet fuel. It is used for the description of the product and to calculate further fuel characteristics. For the international trade of goods it is crucial to precisely know the fuel density or more specifically its apparent density which represents weight by volume.

When large quantities of fuel products are traded, it is not possible to measure the weight of e.g. a whole tanker although the weight of the product is used for invoicing. However, the volume of such a tanker is known and hence the relation of weight to volume (equals apparent density) can be used to calculate the desired parameter (weight) based on the known parameter (volume).
This also works the other way round: when a fuel truck is weighed on a weighbridge before and after it was filled with fuel. The difference between the weights can then be used to calculate the filled volume thanks to the known apparent density measured on a sample by a digital density meter.

Throughout the past 170 years of fuel history common practices in measurement have become standardized differently in different regions. Because density greatly depends on temperature, it is required to report density values at a convenient temperature. To ensure comparability when faced with different measuring temperatures the API conversion was developed. This allows you to perform measurements at any desired temperature (which can be higher than the reporting temperature in the case of crude oil or lower in the case of jet fuel) and to convert the measured density afterwards to the density at a defined reference temperature by using the correct API group. This saves time and paves the way to international comparability. Modern digital density meters, for example, measure at the desired measuring temperature and automatically convert the density reading to the density at a desired reference temperature. Pycnometers and hydrometers require manual conversion of the results using tables.

Table 1: Overview of fuel measuring methods with pros and cons

The density of fuels is typically given in kg/m³. 

Density measurement has to be standardized to make sure that two independent parties that check the fuel density of the same liquid but at different locations can compare results. For fuel density, digital density measurement is basically covered by two standards: ASTM D7777 and ASTM D4052. ASTM D7777 describes the use of portable density meters (also called digital hydrometers) and requires a precision of 1 kg/m³ whereas ASTM D4052 covers fuel density measurements by means of digital density meters in the laboratory and requires a 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.

The procedure for measuring the density of fuels using pycnometers is described in:

ASTM D1217 - Standard Test Method for Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer

The procedure for measuring the density of fuels using hydrometers is described in:

ASTM D1298 - Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method

• How much time and money you can save by using a digital density meter
• The latest developments in digital density measurement
• The fundamentals of density and density measurement