What is Density ?

 Density is the mass of a substance divided by its volume. By the viewpoint of commerce, the knowledge as to the density of fuel oils is fundamentally l important as fuel is delivered by volume and sold by mass. The relationship between mass and volume is as follows;

    • The unit for density is kg/m3 and for fuel a reference temperature is always stated, which is usually 15oC. A reference temperature has to be given because the density of fuel varies with temperature.
    • The terms "density in vacuo" and "density in air" are sometimes found on bunker receipts. Density is the relationship of the mass of a substance to its volume, not its weight to volume ratio, and therefore density by definition is in vacuo. The term "density in air", although often used is incorrect, should be referred to as a "weight factor". This is because a substance weighed in air is supported to a small extent by the buoyancy of the air acting on it, hence the weight of a liquid in air is slightly less than the weight in vacuo. The relationship between density and the corresponding weight factor (incorrectly called "density in air") is not easy to make clear
    • For bunker fuels with a density range of 800 to 1010 kg/m3 at 15oC, the conversion calculation approximates to a difference of 1.1 kg/m3. Hence to convert density at 15oC to the weight factor at 15oC, 1.1kg/m3 should be deducted.
    • Specific gravity is the ratio of the mass of a given volume of a substance to the mass of an equal volume of water at the same temperature. As it is a ratio there are no units but as temperature is stated, eg. 15 / 15oC. Sometimes specific gravity is quoted at 20/4oC, but this is not specific gravity as the temperatures are not identical. It is however relative density, which is the ratio of the mass of a given volume of substance at a temperature t1, to the mass of an equal volume of pure water at temperature t1. Since 1m3 of pure water at 4oC has a mass of 1000kg, the density of a substance at t1oC is equivalent to the relative density at t1 / 4oC.