Editing Relative density (section)

==Temperature dependence==
:''See [[Density]] for a table of the measured densities of water at various temperatures.''

The density of substances varies with temperature and pressure so that it is necessary to specify the temperatures and pressures at which the densities or masses were determined. It is nearly always the case that measurements are made at nominally 1 atmosphere (101.325&nbsp;kPa ignoring the variations caused by changing weather patterns) but as relative density usually refers to highly incompressible aqueous solutions or other incompressible substances (such as petroleum products) variations in density caused by pressure are usually neglected at least where apparent relative density is being measured. For true (''in vacuo'') relative density calculations air pressure must be considered (see below). Temperatures are specified by the notation (''T<sub>s</sub>''/''T<sub>r</sub>'') with ''T<sub>s</sub>'' representing the temperature at which the sample's density was determined and ''T<sub>r</sub>'' the temperature at which the reference (water) density is specified. For example, SG (20&nbsp;°C/4&nbsp;°C) would be understood to mean that the density of the sample was determined at 20&nbsp;°C and of the water at 4&nbsp;°C. Taking into account different sample and reference temperatures, while SG<sub>H<sub>2</sub>O</sub> = 1.000000 (20&nbsp;°C/20&nbsp;°C) it is also the case that RD<sub>H<sub>2</sub>O</sub> = {{sfrac|0.9982008|0.9999720}} = 0.9982288 (20&nbsp;°C/4&nbsp;°C). Here temperature is being specified using the current [[ITS-90]] scale and the densities<ref name="PTB-Mitteilungen">{{cite journal |last1=Bettin |first1=H. |last2=Spieweck |first2=F. |title=Die Dichte des Wassers als Funktion der Temperatur nach Einführung des Internationalen Temperaturskala von 1990 |journal=[[PTB-Mitteilungen]] |volume=100 |year=1990 |pages=195–196 |language=de}}</ref> used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale the densities<ref name="„kell" /> at 20&nbsp;°C and 4&nbsp;°C are, respectively, 0.9982041 and 0.9999720 resulting in an RD (20&nbsp;°C/4&nbsp;°C) value for water of 0.99823205.

The temperatures of the two materials may be explicitly stated in the density symbols; for example:
:relative density: 8.15{{su|b=4&nbsp;°C|p=20&nbsp;°C}}; or specific gravity: 2.432{{su|b=0|p=15}}
where the superscript indicates the temperature at which the density of the material is measured, and the subscript indicates the temperature of the reference substance to which it is compared.