## Applications of Specific Gravity

The following formula is used to calculate the specific gravity of a material.

SG = ρ / ρW

Where SG = specific gravity, ρ = density of the material (kg/m^{3}), ρW = density of water ( kg/m^{3}). The reference density of water at 4^{o}C (39^{o}F) is used as the reference as these are the conditions of maximum density.

There is a wide range of instruments designed to measure the specific gravity of a material. The hydrometer can be used to measure the specific gravity of any liquid. The device is designed to float freely at the liquid surface with a protruding stem giving a reading corresponding to the specific gravity of the liquid.

Specific Gravity has a wide range of applications including:

- Pharmaceuticals: The specific gravity is used to find out the purity of a drug since each of the constituents has a distinct specific gravity.
- Determining the additives used in a base material: Specific gravity is used to find out the amount of additives used in a base material which might affect the performance and stability of the base material.
- Urine Specific Gravity: The Urine Specific gravity (USG) is measured and used mostly in veterinary sciences to understand whether water is excreted or conserved in an appropriate fashion.
- Calculating volume of a fluid: The fluid’s volume can be calculated using the specific gravity of the fluid and the weight. Conversely the weight can be calculated if the volume is known.
- Other applications of specific gravity include fluid mechanics, buoyancy and the brewing industry.

A fluid is a continuous medium; i.e., a substance

that is continuously distributed throughout a region in space. Because

a fluid is a continuous medium, it would be rather awkward to analyze

the fluid as a single entity with a total mass, m,total weight,

W, or total volume, V.

It is more convenient to analyze

the fluid in terms of the mass of fluid contained in a specified

volume. is defined as mass

per unit volume. Density is a property that applies to solids as

well as fluids.

The most commonly used units for density are kg/m^{3}

in the SI system and slug/ft^{3} in the English system. Values

for density can vary widely for different fluids.

For example, the

densities of water and air at 4�C and 1 atm pressure are

approximately 1000 kg/m^{3} (1.94 slug/ft^{3}) and

1.27 kg/m^{3} (0.

00246 slug/ft^{3}), respectively.

Densities of liquids are higher than those of gases because the

intermolecular spacing is smaller. Physical properties vary with

temperature and pressure to some extent.

For liquids, density does not

vary significantly with changes in temperature and pressure, but the

densities of gases are strongly influenced by changes in temperature

and pressure.

A fluid property that is similar to density is . Specific weight is defined as weight per unit

volume. The mathematical definition for specific weight, <img src="https://www.eng.fsu.

The most commonly used units for specific weight

are N/m^{3} in the SI system and lb_{f}/ft^{3}

in the English system. Note that the unit for specific weight in the

English system is not lb_{m}/ft^{3}.

The unit lb_{m}

is a unit of mass, not a unit of weight. A quick inspection of

Equation 7-1 and Equation 7-2 reveals that specific weight is

essentially the same property as density with mass replaced by weight.

A formula that relates density, ,

and specific weight, <img src="https://www.eng.fsu.

edu/~chandra/courses/eml3004c/book/Chapter13/Images/gamma.gif» border=»0″ width=»9″ height=»13″>,

may be obtained by noting that the weight of a unit volume of fluid is

W=mg, where g is the local gravitational

acceleration.

Using the standard value of gravitational

acceleration, g=9.807 m/s^{2}, water at 4�C has a

specific weight of

Doing the same calculation in English units, noting

that the standard value of gravitational acceleration is g=32.174

ft/s^{2}, water at 4�C (39.2�F) has a specific weight of

The rationale for finding the density and specific

weight of water at 4�C in the foregoing discussion is that 4�C is a

reference temperature on which is based. Specific gravity is defined as the ratio

of the density of a fluid to the density of water at a reference

temperature.

Typically, the reference temperature is taken as 4�C

because the density of water is maximum (about 1000 kg/m^{3})

at this temperature. The mathematical definition for specific gravity,

sg, is

Because specific gravity is a ratio of two

properties with the same units, it is a dimensionless quantity.

Furthermore, the value of sg does not depend on the system of

units used.

Using English units, we obtain the same value.

Specific gravity may also be defined as the ratio

of the specific weight of a fluid to the specific weight of water at a

reference temperature. This definition, which is derived by

combining Equation 7-4 and Equation 7-3, is expressed as