Density is the ratio of the mass of a substance to its volume. Absolute density is usually expressed as kg/m³ but may also be quoted as kg/litre and as g/litre. Relative density (or specific gravity - SG) is the comparison of the density of a substance with the density of freshwater - both at quoted reference temperatures - and is a dimensionless ratio. The density value is required for the selection of the correct purifier gravity disc.
Commercially, density is required to convert product delivery volumes into masses and vice versa.