Definition

specific gravity

The term specific gravity, symbolized sp gr, refers to the ratio of the density of a solid or liquid to the density of water at 4 degrees Celsius. The term can also refer to the ratio of the density of a gas to the density of dry air at standard temperature and pressure, although this specification is less often used. Specific gravity is a dimensionless quantity; that is, it is not expressed in units.

To find the sp gr of a solid or liquid, you must know its density in kilograms per meter cubed (kg/m3) or in grams per centimeter cubed (g/cm3). Then, divide this density by the density of pure water in the same units. If you use kg/m3, divide by 1000. If you use g/cm3, divide by 1 (that is, leave the number alone). It is important to use the same units in the numerator and denominator.

Water has a specific gravity equal to 1. Materials with a specific gravity less than 1 are less dense than water, and will float on the pure liquid; substances with a specific gravity more than 1 are more dense than water, and will sink. An object with a density of 85 kg/m3 has a specific gravity of 0.085, and will float high on the surface of a body of water. An object with a density of 85 g/cm3 has a specific gravity of 85, and will sink rapidly.

To find the specific gravity of a gas, you must know its density in kilograms per meter cubed (kg/m3). Then, divide this density by the density of dry air at standard temperature and pressure. This value is approximately 1.29 kg/m3. Gases with a specific gravity less than 1 will rise in the atmosphere at sea level; gases with a specific gravity greater than 1 will sink and seek regions of low elevation at the earth's surface.

Also see kilogram, meter, kilogram per meter cubed, SI (International System of Units), and standard temperature and pressure.

This was last updated in September 2005
Posted by: Margaret Rouse

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