Demystifying Specific Gravity and Its Relation to Density in Process Engineering

Specific gravity is a relative measure that compares the density of a substance to the density of a reference substance, usually water at a certain temperature. Density is an absolute measure that tells us how much mass is contained in a given volume of a substance.

To calculate specific gravity, you need to know the density of the substance and the density of water, and then divide them. For example, if the density of a substance is 1200 kg/m³ and the density of water is 1000 kg/m³, then the specific gravity of the substance is 1200/1000 = 1.2.

To calculate density, you need to know the mass and the volume of the substance, and then divide them. For example, if the mass of a substance is 300 g and the volume is 250 cm³, then the density of the substance is 300/250 = 1.2 g/cm³.

Specific gravity and density are related, but they are not the same. Specific gravity is a dimensionless number, while density has units of mass per unit volume. Specific gravity can tell us if a substance will float or sink in water, while density can tell us how compact or sparse a substance is. Specific gravity is also affected by the temperature and pressure of the substance and the reference substance, while density is only affected by the temperature and pressure of the substance.

Relationship Between Specific Gravity and Density

The relationship between specific gravity (SG) and density (?) can be mathematically expressed as:

Specific Gravity (SG) = $\frac{{\text{Density of substance}}}{{\text{Density of water at specified temperature}}}$

SG = $\frac{{\rho_{\text{substance}}}}{{\rho_{\text{water}}}}$

Where:

$\rho_{\text{substance}}$ = Density of the substance
$\rho_{\text{water}}$ = Density of water at specified temperature

Example Scenario and Calculation

Let’s consider a scenario in a chemical processing plant where a liquid with a specific gravity of 0.85 needs to be analyzed for its density.

Given Data:
- Specific Gravity (SG) of the liquid = 0.85
- Density of water at 20°C = 998 kg/m³
Calculation:Using the relationship between specific gravity and density:

Density of substance = SG × Density of water at specified temperature

$\rho_{\text{substance}} = \text{SG} \times \rho_{\text{water}}$

$\rho_{\text{substance}} = 0.85 \times 998 \, \text{kg/m}^3$

$\rho_{\text{substance}} = 848.3 \, \text{kg/m}^3$
Result:The density of the liquid is calculated to be 848.3 kg/m³.