What is Specific Gravity?

Specific gravity is a special case of relative density. In this case the density of a substance is divided by the density of water at 4 F (0 C). Since water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is the same number as density but without any units (adimensional).

Relative density is another adimensional number which measures density of any substance in relation to another. A relative density greater than one means that the substance in the numerator has a bigger density than the one in the denominator and vice versa.

Applying the same concept to specific gravity: if the substance is not miscible with water (do not mix or dissolve) it will float for specific density over one and will sink for specific gravity under one.

Density is the ratio between mass and volume. It reveals interesting features of a substance and is essentially related with the way the atoms and molecules in the substance are arranged. It will depend on the temperature among other things.

Specific Gravity Determination

Liquids: The specific gravity of a liquid can be determined with a hydrometer. The depth to which the hydrometer sinks is inversely proportional to the specific gravity of the liquid. A hydrometer is a hollow, sealed, calibrated glass tube.

Solids: Some electronic scales can measure specific gravity of solids. This is particularly useful for purity determination in gems.

Gases: The specific gravity of gases is measured by specific gravity transducers. They are particularly useful for power, oil, gas, aerospace and process industries.

Practical uses of Specific Gravity

Specific gravity measurements can be used in a wide variety of industries. It is particularly useful because it allows access to molecular information in a non invasive way.

Purity in gems: comparing the specific gravity of a gem with the patron number (measured over a gem with high purity level), the degree of purity of a gem can be observed. This allows for fast determinations of the value of a gem.

Eggshell thickness: The eggshell thickness is an important factor in the poultry industry; it will decay with the age of the hen or under adverse conditions. If the thickness goes under certain expected values, the eggs are not marketable. The specific gravity of an egg is determined mainly by its shell, the other components have specific gravity close to one (shell= 2.325; yolk= 1.032; albumen= 1.038; shell membranes= 1.075). Measuring the specific gravity of the egg as a whole one has a good idea of the state of the shell.

Compressive strength of soils: Engineers need to know the compressive strength of a soil to choose among other things the construction method. Specific gravity and compressibility are highly connected and can be obtained one from the other.

Oil Industry: Crude oil and its refined products are normally measured either by volume in gallons and US barrels, or by weight in tons. The relationship between volume and weight can be measured by specific gravity or density. The specific gravity is related with the "degree API", a measure of the inherent power of the oil (regular, super, etc).

Table of Specific Gravities used in the Oil Industry

Product	Specific Gravity Ranges
Crude oils	0.80-0.97
Aviation gasolines	0.70-0.78
Motor gasolines	0.71-0.79
Kerosines	0.78-0.84
Gas oils	0.82-0.90
Diesel oils	0.82-0.92
Lubricating oils	0.85-0.95
Fuel oils	0.92-0.99
Asphaltic bitumens	1.00-1.10