Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass divided by volume) of a substance to the density of a given reference material. The relative density of 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 (25 °C or 77.0 °F). The term "relative density" (abbreviated r.d. or RD) is preferred modern use, for example in ISO, IUPAC, and NIST whereas the term "specific gravity" (abbreviated S.G. or SG) is gradually being abandoned. 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 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. The pressure is nearly always atmospheric pressure (1 atm or 101.325 kPa), and where it is not, usually the density is specified directly. The reference temperature for water is often 4 °C (39.2 °F), but 15 °C (59.0 °F) and 20 °C (68.0 °F) are also common standards, depending on the industry (like brewing or petroleum). In British brewing practice, the relative density, as specified above, is multiplied by 1000. Relative density 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.
Basic calculation Relative density ( R D {\displaystyle \mathrm {RD} } ) or specific gravity ( S G {\displaystyle \mathrm {SG} } ) is a dimensionless quantity, as it is the ratio of either densities or weights
R D = ρ s u b s t a n c e ρ r e f e r e n c e , {\displaystyle \mathrm {RD} ={\frac {\rho _{\mathrm {substance} }}{\rho _{\mathrm {reference} }}},}
where R D {\displaystyle \mathrm {RD} } is relative density, ρ s u b s t a n c e {\displaystyle \rho _{\mathrm {substance} }} is the density of the substance being measured, and ρ r e f e r e n c e {\displaystyle \rho _{\mathrm {reference} }} is the density of the reference. (By convention ρ {\displaystyle \rho } , the Greek letter rho, denotes density.) The reference material can be indicated using subscripts: R D s u b s t a n c e / r e f e r e n c e {\displaystyle \mathrm {RD} _{\mathrm {substance/reference} }} which means "the relative density of substance with respect to reference". If the reference is not explicitly stated then it is normally assumed to be water at 4 °C (or, more precisely, 3.98 °C, which is the temperature at which water reaches its maximum density). In SI units, the density of water is (approximately) 1000 kg/m3 or 1 g/cm3, which makes relative density calculations particularly convenient: the density of the object only needs to be divided by 1000 or 1, depending on the units. The relative density of gases is often measured with respect to dry air at a temperature of 20 °C and a pressure of 101.325 kPa absolute, which has a density of 1.205 kg/m3. Relative density with respect to air can be obtained by
R D = ρ g a s ρ a i r ≈ M g a s M a i r , {\displaystyle {\mathit {RD}}={\frac {\rho _{\mathrm {gas} }}{\rho _{\mathrm {air} }}}\approx {\frac {M_{\mathrm {gas} }}{M_{\mathrm {air} }}},}
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