What is Specific Weight ?

Specific weight compares the weight per unit volume, while density compares the mass per unit volume. In other words, specific weight accounts for gravity. While density is absolute, specific weight depends on factors like gravitational acceleration, which varies depending on altitude and latitude.

Gravitational acceleration is higher at the poles than at the equator. Gravity also decreases slightly as altitude increases because you move away from the Earth’s center of mass.

Specific weight and specific gravity are connected. Specific gravity compares the specific weight of an air or gas to that of a standard at the same pressure and temperature. If you know the specific gravity of a substance, you can find its specific weight.

You can determine specific weight by multiplying density by the acceleration due to gravity. Here’s the formula and what each part means:

γ = pg

• γ: Specific weight (gamma) in newtons per meter cubed (N/m³)
• p: Density in kilograms per meter cubed (kg/m³)
• g: Gravitational acceleration in meters per second squared (m/s²)

If you wanted to figure out the specific weight of air, you could use density and gravitational acceleration. For example, the standard density of air at sea level is about 1.225 kg/m³ at 15^°C, and the standard gravitational acceleration is about 9.81 m/s². When you multiply these two values, you get a specific weight of about 12.01 N/m³.

Unlike density, specific weight is not constant and changes depending on elevation and temperature. However, here are the standards typically given for the two substances:

• Water: Its specific weight at standard sea-level atmospheric pressure and 15^°C is about 9,797.8 N/m³.
• Air: The specific weight of air at standard sea-level atmospheric pressure and 15^°C is about 12.01 N/m³, as shown in the example calculation for the previous question.

Here are a few particular applications of specific weight for industrial air compressors:

• System sizing: Specific weight helps determine pneumatic systems’ volume and pressure requirements, ensuring a compressor delivers the right amount of air.
• Structural calculations: In large-scale compressor installations, you need to know the specific weight to determine the air load on tanks, pipes and supports.
• Fluid mechanics: Specific weight is essential for understanding how air moves, which helps you determine the force needed to move materials in systems like conveyors.