Specific gravity

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Not to be confused with specific weight.

Specific gravity is defined as the ratio of the density of a given substance to the density of water, when both are at the same temperature, it is therefore a dimensionless quantity (see below). Substances with a specific gravity greater than one are denser than water, and so (ignoring surface tension effects) will sink in it, and those with a specific gravity of less than one are less dense than water, and so will float in it. Specific gravity is a special case of, or in some usages synonymous with, relative density, with the latter term often preferred in modern scientific writing. The use of specific gravity is discouraged in technical use in scientific fields requiring high precision — actual density (in dimensions of mass per unit volume) is preferred.

Specific gravity, SG, is expressed mathematically as:

\mbox{SG} = \frac{\rho_\mathrm{substance}}{\rho_{\mathrm{H}_2\mathrm{O}}}

where \rho_\mathrm{substance}\, is the density of the substance, and \rho_{\mathrm{H}_2\mathrm{O}} is the density of water. (By convention ρ, the Greek letter rho, denotes density.) The density of water varies with temperature and pressure, and it is usual to refer specific gravity to the density at 4°C (39.2°F) and a normal pressure of 1 atm. In this case \rho_{\mathrm{H}_2\mathrm{O}} is equal to 1000 kg·m−3 in SI units (or 62.43 lb·ft−3 in United States customary units).

Given the specific gravity of a substance, its actual density can be calculated by inverting the above formula:

{\rho_\mathrm{substance}} = \mbox{SG} \times \rho_{\mathrm{H}_2\mathrm{O}}

Occasionally a reference substance other than water is specified (for example, air), in which case specific gravity means density relative to that reference.

Specific gravity is by definition dimensionless and therefore not dependent on the system of units used (e.g. slugs·ft−3 or kg·m−3). However, the two densities must of course be converted to the same units before carrying out the numerical ratio calculation.

For information about the measurement of and uses of specific gravity, see relative density.

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[edit] Examples

  • Balsa wood has a specific gravity of 0.2, so it is 0.2 times as dense as water.
  • Aluminium has a specific gravity of 2.7, so it is 2.7 times as dense as water.
  • Lead has a specific gravity of 11.35, so it is 11.35 times as dense as water.

(Samples may vary, and these figures are approximate.)

[edit] See also

[edit] External links

[edit] Books

  • Fundamentals of Fluid Mechanics Wiley, B.R. Munson, D.F. Young & T.H. Okishi
  • Introduction to Fluid Mechanics Fourth Edition, Wiley, SI Version, R.W. Fox & A.T. McDonald
  • Thermodynamics: An Engineering Approach Second Edition, McGraw-Hill, International Edition, Y.A. Cengel & M.A. Boles