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.