SPECIFIC GRAVITY

Specific gravity (sp. gr.) is the ratio of specific weight (or mass density) of a fluid to the specific weight (or mass density) of a standard fluid. For liquids, the standard fluid chosen for comparison is pure water at 4°C (39.2°F). For gases, the standard fluid chosen is either hydrogen or air at some specified temperature and pressure. As specific weight and mass density of a fluid vary with temperature, temperatures must be quoted when specific gravity is used in precise calculations of specific weight or mass density. Being a ratio of two quantities with same units, specific gravity is a pure number independent of the system of units used. The specific gravity of water at the standard temperature (i.e., 4°C), is therefore, equal to 1.0. The specific gravity of mercury varies from 13.5 to 13.6. Knowing the specific gravity of any liquid, its specific weight may be readily calculated by the following relation,

    w = Sp. gr. of liquid × Specific weight of water

    = (Sp. gr. of liquid) × 9 810 N/m³.

MATERIAL	SPECIFIC GRAVITY
DRY AIR	0.0013
ALCOHOL	0.82
CARBON DIOXIDE	0.00126
CAST IRON	7.20
PETROL	0.72
RUBBER	0.96
WOOD OAK	0.77

Example 1. If 5 m3 of a certain oil weighs 4000 kg(f). Calculate the specific weight, mass density and specific gravity of this oil.