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} :''This page is about the measurement using water as a reference. For a general use of specific gravity, see relative density. See intensive property for the property implied by "specific". Specific gravity is the ratio of the density of a substance to the density of a reference substance; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume. ''Apparent'' specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water at its densest (4°C) for liquids and for gases it is air at room temperature (21°C). Nonetheless, the temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa). Temperatures for both sample and reference vary from industry to industry. In British beer brewing, the practice for specific gravity as specified above is to multiply it by 1000.〔Hough, J.S., Briggs, D.E., Stevens, R and Young, T.W. Malting and Brewing Science, Vol. II Hopped Wort and Beer, Chapman and Hall, London, 1991, p. 881〕 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, hydrocarbons, sugar solutions (syrups, juices, honeys, brewers wort, must etc.) and acids. ==Details== Being a ratio of densities, specific gravity is a dimensionless quantity. Specific gravity varies with temperature and pressure; reference and sample must be compared at the same temperature and pressure or be corrected to a standard reference temperature and pressure. Substances with a specific gravity of 1 are neutrally buoyant in water. Those with SG greater than 1 are denser than water and will, disregarding surface tension effects, sink in it. Those with an SG less than 1 are less dense than water and will float on it. In scientific work, the relationship of mass to volume is usually expressed directly in terms of the density (mass per unit volume) of the substance under study. It is in industry where specific gravity finds wide application, often for historical reasons. True specific gravity can be expressed mathematically as: : where is the density of the sample and is the density of water. The apparent specific gravity is simply the ratio of the weights of equal volumes of sample and water in air: : where the weight of water, both measured in air. It can be shown that true specific gravity can be computed from different properties: : where is the local acceleration due to gravity, is the volume of the sample and of water (the same for both), is the density of water and represents a weight obtained in vacuum. The density of water varies with temperature and pressure as does the density of the sample. So it is necessary to specify the temperatures and pressures at which the densities or weights were determined. It is nearly always the case that measurements are made at 1 nominal atmosphere (1013.25 mbar ± variations from changing weather patterns). But as specific gravity 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 specific gravity is being measured. For true (''in vacuo'') specific gravity calculations, air pressure must be considered (see below). Temperatures are specified by the notation with representing the temperature at which the sample's density was determined and the temperature at which the reference (water) density is specified. For example, SG (20°C/4°C) would be understood to mean that the density of the sample was determined at 20°C and of the water at 4°C. Taking into account different sample and reference temperatures, we note that, while (20°C/20°C), it is also the case that (20°C/4°C). Here, temperature is being specified using the current ITS-90 scale and the densities〔Bettin, H.; Spieweck, F.: "Die Dichte des Wassers als Funktion der Temperatur nach Einführung des Internationalen Temperaturskala von 1990" PTB-Mitteilungen 100 (1990) pp. 195–196〕 used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale, the densities at 20 °C and 4 °C are 0.9982071 and 0.9999720 respectively, resulting in an SG (20°C/4°C) value for water of 0.9982343. As the principal use of specific gravity measurements in industry is determination of the concentrations of substances in aqueous solutions and as these are found in tables of SG vs concentration, it is extremely important that the analyst enter the table with the correct form of specific gravity. For example, in the brewing industry, the Plato table lists sucrose concentration by weight against true SG, and was originally (20°C/4°C)〔ASBC Methods of Analysis Preface to Table 1: Extract in Wort and Beer, American Society of Brewing Chemists, St Paul, 2009〕 i.e. based on measurements of the density of sucrose solutions made at laboratory temperature (20 °C) but referenced to the density of water at 4 °C which is very close to the temperature at which water has its maximum density equal to 0.999972 g·cm−3 in SI units (or 62.43 lbm·ft−3 in United States customary units). The ASBC table〔ASBC Methods of Analysis ''op. cit.'' Table 1: Extract in Wort and Beer〕 in use today in North America, while it is derived from the original Plato table is for apparent specific gravity measurements at (20°C/20°C) on the IPTS-68 scale where the density of water is 0.9982071 g·cm−3. In the sugar, soft drink, honey, fruit juice and related industries sucrose concentration by weight is taken from a table prepared by A. Brix which uses SG (17.5°C/17.5°C). As a final example, the British SG units are based on reference and sample temperatures of 60F and are thus (15.56°C/15.56°C). Given the specific gravity of a substance, its actual density can be calculated by rearranging the above formula: : Occasionally a reference substance other than water is specified (for example, air), in which case specific gravity means density relative to that reference. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「specific gravity」の詳細全文を読む スポンサード リンク
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