Specific Gravity (Relative Density) SG also called relative density that is ratio of the density of a substance to that of a standard substance. At a specified temperature and can be expressed as

SG = ρsubstance / ρH2O

where

SG = Specific Gravity of the substance

ρsubstance = density of the fluid or substance [kg/m3]

ρH2O = density of water – normally at temperature 4 oC [kg/m3]

The specific gravity of a gas, γ, is the ratio of the density of the gas at standard pressure and temperature to the density of air at the same standard pressure and temperature. The standard temperature is usually 60°F, and the standard pressure is usually 14.696 psia. However, slightly different standards are sometimes used in different locations and in different units. The ideal gas laws can be used to show that the specific gravity (ratio of densities) is also equal to the ratio of the molecular weights. By convention, specific gravities of all gases at all pressures are usually set equal to the ratio of the molecular weight of the gas to that of air (28.967). Although specific gravity is still frequently used, this traditional term is not used under the SI system; it has been replaced by “relative density.”

### Density of Water

The density of water is a crucial parameter in many scientific and engineering applications. Density is defined as mass per unit volume, and for water, it varies slightly depending on temperature and pressure conditions.

At room temperature (approximately 20°C or 68°F) and standard atmospheric pressure, the density of water is about 998 kg/m³ (kilograms per cubic meter) or 62.4 pcf (pounds per cubic foot).

### Specific Gravity of Steam

Specific gravity is a dimensionless quantity that compares the density of a substance to the density of a reference substance, usually water. In the case of steam, the specific gravity is typically determined at a specific temperature and pressure. As steam is less dense than water, its specific gravity is less than 1.

### Difference Between Specific Gravity and Density

While specific gravity and density are related, they are not synonymous. Density is a measure of mass per unit volume for a substance, while specific gravity is a ratio comparing the density of a substance to the density of a reference substance, often water. In essence, specific gravity is a relative measure that allows us to compare the densities of different substances without considering their specific units.

### Relation Between Density and Specific Gravity

The relationship between density and specific gravity can be expressed using the following formula:

Specific Gravity (Sp. Gravity) = (Density of Substance) / (Density of Reference Substance)

For instance, if we want to determine the specific gravity of natural gas compared to air, we would divide the density of natural gas by the density of air.

### Specific Gravity of Natural Gas and Hot Water

The specific gravity of natural gas varies depending on its composition, but it typically ranges between 0.6 and 0.7 when compared to air as the reference substance. Hot water, on the other hand, has a lower density than cold water due to thermal expansion, resulting in a specific gravity slightly less than 1 when compared to water at room temperature.

### Unit Volume and Density Values

Density is expressed in various units, such as kilograms per cubic meter (kg/m³) or pounds per cubic foot (pcf). The choice of unit depends on the application and the system of units being used. It is essential to be aware of the appropriate conversion factors when working with different density measurement units.

### Specific Gravity Decimal Ratio and Same Numerical Value

Specific gravity is a dimensionless number, often represented as a decimal ratio. When comparing two substances with identical densities, their specific gravity will have the same numerical value. For example, if two liquids have the same density as water, their specific gravity will both be 1.

### Calculating Specific Gravity

To calculate specific gravity, divide the density of the substance by the density of the reference substance. This can be done using a specific gravity calculator, which simplifies the process and ensures accurate results.