Relative density

Specific gravity
Common symbols
SG
SI unitUnitless
Derivations from
other quantities
A United States Navy Aviation boatswain's mate tests the specific gravity of JP-5 fuel

Relative density, also called specific gravity,[1][2] is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nearly always measured with respect to water at its densest (at 4 °C or 39.2 °F); for gases, the reference is air at room temperature (20 °C or 68 °F). The term "relative density" (abbreviated r.d. or RD) is preferred in SI, whereas the term "specific gravity" is gradually being abandoned.[3]

If a substance's relative density is less than 1 then it is less dense than the reference; if greater than 1 then it is denser than the reference. If the relative density is exactly 1 then the densities are equal; that is, equal volumes of the two substances have the same mass. If the reference material is water, then a substance with a relative density (or specific gravity) less than 1 will float in water. For example, an ice cube, with a relative density of about 0.91, will float. A substance with a relative density greater than 1 will sink.

Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa). Where it is not, it is more usual to specify the density directly. Temperatures for both sample and reference vary from industry to industry. In British brewing practice, the specific gravity, as specified above, is multiplied by 1000.[4] Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as brines, must weight (syrups, juices, honeys, brewers wort, must, etc.) and acids.

  1. ^ Dana, Edward Salisbury (1922). A text-book of mineralogy: with an extended treatise on crystallography... New York, London(Chapman Hall): John Wiley and Sons. pp. 195–200, 316.
  2. ^ Schetz, Joseph A.; Allen E. Fuhs (1999-02-05). Fundamentals of fluid mechanics. Wiley, John & Sons, Incorporated. pp. 111, 142, 144, 147, 109, 155, 157, 160, 175. ISBN 0-471-34856-2.
  3. ^ United States Bureau of Reclamation (1978). Metric Manual. U.S. Department of the Interior, Bureau of Reclamation. p. 37.
  4. ^ Cite error: The named reference briggs was invoked but never defined (see the help page).