Relative density, or specific gravity,^{[1]}^{[2]} is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water. The term "relative density" is often preferred in modern scientific usage.
If a substance's relative density is less than one then it is less dense than the reference; if greater than one then it is denser than the reference. If the relative density is exactly one 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 one 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 one will sink.
Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm equal to 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.^{[3]} 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, sugar solutions (syrups, juices, honeys, brewers wort, must, etc.) and acids.
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Relative density (RD) or specific gravity (SG) is a dimensionless quantity, as it is the ratio of either densities or weights
where RD is relative density, ρ_{substance} is the density of the substance being measured, and ρ_{reference} is the density of the reference. (By convention ρ, the Greek letter rho, denotes density.)
The reference material can be indicated using subscripts: RD_{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/m^{3} or 1 g/cm^{3}, 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/m^{3}. Relative density with respect to air can be obtained by
Where M is the molar mass and the approximately equal sign is used because equality pertains only if 1 mol of the gas and 1 mol of air occupy the same volume at a given temperature and pressure i.e. they are both Ideal gasses. Ideal behaviour is usually only seen at very low pressure. For example, one mol of an ideal gas occupies 22.414 L at 0 °C and 1 atmosphere whereas carbon dioxide has a molar volume of 22.259 L under those same conditions.
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 weights were determined. It is nearly always the case that measurements are made at nominally 1 atmosphere (101.325 kPa the variations caused by 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 T_{s}/T_{r}) with T_{s} representing the temperature at which the sample's density was determined and T_{r} 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 SG_{H}2O = 1.000000 (20°C/20°C) it is also the case that SG_{H}2O = 0.998203/0.998840 = 0.998363 (20°C/4°C). Here temperature is being specified using the current ITS90 scale and the densities^{[4]} used here and in the rest of this article are based on that scale. On the previous IPTS68 scale the densities at 20 °C and 4 °C are, respectively, 0.9982071 and 0.9999720 resulting in an SG (20°C/4°C) value for water of 0.9982343.
The temperatures of the two materials may be explicitly stated in the density symbols; for example:
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.
Relative density can also help quantify the buoyancy of a substance in a fluid, or determine the density of an unknown substance from the known density of another. Relative density is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemologists use it as an aid in the identification of gemstones. Water is preferred as the reference because measurements are then easy to carry out in the field (see below for examples of measurement methods).
As the principal use of specific gravity measurements in industry is determination of the concentrations of substances in aqueous solutions and 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, which lists sucrose concentration by weight against true SG, were originally (20 °C/4 °C)^{[5]} that is 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 of ρ(H_{2}O) equal to 0.999972 g/cm^{3} (or 62.43 lb_{m}·ft^{−3}). The ASBC table^{[6]} 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 IPTS68 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 this work^{[3]} 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 60°F and are thus (15.56°C/15.56°C).^{[3]}
Relative density can be calculated directly by measuring the density of a sample and dividing it by the (known) density of the reference substance. The density of the sample is simply its mass divided by its volume. Although mass is easy to measure, the volume of an irregularly shaped sample can be more difficult to ascertain. One method is to put the sample in a waterfilled graduated cylinder and read off how much water it displaces. Alternatively the container can be filled to the brim, the sample immersed, and the volume of overflow measured. The surface tension of the water may keep a significant amount of water from overflowing, which is especially problematic for small samples. For this reason it is desirable to use a water container with as small a mouth as possible.
For each substance, the density, ρ, is given by
When these densities are divided, references to the spring constant, gravity and crosssectional area simply cancel, leaving
Relative density is more easily and perhaps more accurately measured without measuring volume. Using a spring scale, the sample is weighed first in air and then in water. Relative density (with respect to water) can then be calculated using the following formula:
where
This technique cannot easily be used to measure relative densities less than one, because the sample will then float. W_{water} becomes a negative quantity, representing the force needed to keep the sample underwater.
Another practical method uses three measurements. The sample is weighed dry. Then a container filled to the brim with water is weighed, and weighed again with the sample immersed, after the displaced water has overflowed and been removed. Subtracting the last reading from the sum of the first two readings gives the weight of the displaced water. The relative density result is the dry sample weight divided by that of the displaced water. This method works with scales that can't easily accommodate a suspended sample, and also allows for measurement of samples that are less dense than water.
The relative density of a liquid can be measured using a hydrometer. This consists of a bulb attached to a stalk of constant crosssectional area, as shown in the diagram to the right.
First the hydrometer is floated in the reference liquid (shown in light blue), and the displacement (the level of the liquid on the stalk) is marked (blue line). The reference could be any liquid, but in practice it is usually water.
The hydrometer is then floated in a liquid of unknown density (shown in green). The change in displacement, Δx, is noted. In the example depicted, the hydrometer has dropped slightly in the green liquid; hence its density is lower than that of the reference liquid. It is, of course, necessary that the hydrometer floats in both liquids.
The application of simple physical principles allows the relative density of the unknown liquid to be calculated from the change in displacement. (In practice the stalk of the hydrometer is premarked with graduations to facilitate this measurement.)
In the explanation that follows,
Since the floating hydrometer is in static equilibrium, the downward gravitational force acting upon it must exactly balance the upward buoyancy force. The gravitational force acting on the hydrometer is simply its weight, mg. From the Archimedes buoyancy principle, the buoyancy force acting on the hydrometer is equal to the weight of liquid displaced. This weight is equal to the mass of liquid displaced multiplied by g, which in the case of the reference liquid is ρ_{ref}Vg. Setting these equal, we have
or just
(1) 
Exactly the same equation applies when the hydrometer is floating in the liquid being measured, except that the new volume is V  AΔx (see note above about the sign of Δx). Thus,
(2) 
Combining (1) and (2) yields
(3) 
But from (1) we have V = m/ρ_{ref}. Substituting into (3) gives
(4) 
This equation allows the relative density to be calculated from the change in displacement, the known density of the reference liquid, and the known properties of the hydrometer. If Δx is small then, as a firstorder approximation of the geometric series equation (4) can be written as:
This shows that, for small Δx, changes in displacement are approximately proportional to changes in relative density.
A pycnometer (from Greek: πυκνός (puknos) meaning "dense"), also called pyknometer or specific gravity bottle, is a device used to determine the density of a liquid. A pycnometer is usually made of glass, with a closefitting ground glass stopper with a capillary tube through it, so that air bubbles may escape from the apparatus. This device enables a liquid's density to be measured accurately by reference to an appropriate working fluid, such as water or mercury, using an analytical balance.
If the flask is weighed empty, full of water, and full of a liquid whose specific gravity is desired, the specific gravity of the liquid can easily be calculated. The particle density of a powder, to which the usual method of weighing cannot be applied, can also be determined with a pycnometer. The powder is added to the pycnometer, which is then weighed, giving the weight of the powder sample. The pycnometer is then filled with a liquid of known density, in which the powder is completely insoluble. The weight of the displaced liquid can then be determined, and hence the specific gravity of the powder.
There is also an airbased manifestation of a pycnometer known as a gas pycnometer; it compares the change in pressure caused by a measured change in a closed volume containing a reference (usually a steel sphere of known volume) with the change in pressure caused by the sample under the same conditions. The difference in change of pressure represents the volume of the sample as compared to the reference sphere, and is usually used for solid particulates that may dissolve in the liquid medium of the pycnometer design described above.
When a pycnometer is filled to a specific, but not necessarily accurately known volume, V and is placed upon a balance, it will exert a force
where m_{b} is the mass of the bottle and g the gravitational acceleration at the location at which the measurements are being made. ρ_{a} is the density of the air at the ambient pressure and ρ_{b} is the density of the material of which the bottle is made (usually glass) so that the second term is the mass of air displaced by the glass of the bottle whose weight, by Archimedes Principle must be subtracted. The bottle is, of course, filled with air but as that air displaces an equal amount of air the weight of that air is canceled by the weight of the air displaced. Now we fill the bottle with the reference fluid e.g. pure water. The force exerted on the pan of the balance becomes:
If we subtract the force measured on the empty bottle from this (or tare the balance before making the water measurement) we obtain.
where the subscript n indicated that this force is net of the force of the empty bottle. The bottle is now emptied, thoroughly dried and refilled with the sample. The force, net of the empty bottle, is now:
where ρ_{s} is the density of the sample. The ratio of the sample and water forces is:
This is called the Apparent Specific Gravity, denoted by subscript A, because it is what we would obtain if we took the ratio of net weighings in air from an analytical balance or used a hydrometer (the stem displaces air). Note that the result does not depend on the calibration of the balance. The only requirement on it is that it read linearly with force. Nor does SG_{A} depend on the actual volume of the pycnometer.
Further manipulation and finally substitution of SG_{V}, the true specific gravity,(the subscript V is used because this is often referred to as the specific gravity in vacuo) for ρ_{s}/ρ_{w} gives the relationship between apparent and true specific gravity.
In the usual case we will have measured weights and want the true specific gravity. This is found from
Since the density of dry air at 101.325 kPa at 20 °C is^{[7]} 0.001205 g/cm^{3} and that of water is 0.998203 g/cm^{3} we see that the difference between true and apparent specific gravities for a substance with specific gravity (20°C/20°C) of about 1.100 would be 0.000120. Where the specific gravity of the sample is close to that of water (for example dilute ethanol solutions) the correction is even smaller.
The pycnometer is used in ISO standard: ISO 11831:2004, ISO 10141985 and ASTM standard: ASTM D854.
In modern laboratories precise measurements of specific gravity are made using oscillating Utube meters. These are capable of measurement to 5 to 6 places beyond the decimal point and are used in the brewing, distilling, pharmaceutical, petroleum and other industries. The instruments measure the actual mass of fluid contained in a fixed volume at temperatures between 0 and 80 °C but as they are microprocessor based can calculate apparent or true specific gravity and contain tables relating these to the strengths of common acids, sugar solutions, etc.
Substances with a specific gravity of 1 are neutrally buoyant, those with SG greater than one are denser than water, and so (ignoring surface tension effects) will sink in it, and those with an SG of less than one are less dense than water, and so will float.
(Samples may vary, and these figures are approximate.)
Specific gravity is defined as the ratio of the density of a given solid or liquid substance to the density of water at a specific temperature and pressure, typically at 4 °C (39 °F) and 1 atm (760.00 mmHg).
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Specific gravity is 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 applications requiring high precision. Actual density, in dimensions of mass per unit volume, is preferred in such cases.
Specific gravity, SG, is expressed mathematically as:
where $\backslash rho\_\backslash mathrm\{substance\}\backslash ,$ is the density of the substance, and $\backslash rho\_\{\backslash mathrm\{H\}\_2\backslash mathrm\{O\}\}$ 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. The given temperature and pressure are preferred because it is when water has its maximum density. In this case $\backslash rho\_\{\backslash mathrm\{H\}\_2\backslash mathrm\{O\}\}$ is equal to 1000 kg·m^{−3} in SI units (or 62.43 lb_{m}·ft^{−3} in United States customary units).
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.
Specific gravity is, by definition, dimensionless and therefore independent on the system of units used (e.g. slugs·ft^{−3} or kg·m^{−3}). However, the two densities must 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.
(Samples may vary, and these figures are approximate.)

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Specific Gravity (SG) contributes to relative density.
It is defined as the ratio of the density of a given substance, to the density of water (H_{2}O). Substances with a specific gravity greater than 1 are heavier than water, and those with a specific gravity of less than 1 are lighter than water.
$\backslash mbox\{SG\}\; =\; \backslash frac\{\backslash rho\_\backslash mathrm\{substance\}\}\{\backslash rho\_\backslash mathrm\{H2O\}\; \}$ ^{[1]}
SG is by definition dimensionless and therefore not dependent on the system of units used (e.g. slugsft^{3} or kgm^{3}), insofar as the units are consistent. Based on the SGvalue of a given substance, the density of that substance can be calculated.
