Density, specific weight, specific gravity, and compressibility are fundamental properties of fluids. Density is the mass per unit volume, characterizing the mass of a fluid system. It influences buoyancy, pressure, flow dynamics, viscosity, thermal conductivity, and sound propagation. For instance, in pipeline design, accurate density measurements ensure that the pipeline can handle the fluid's mass.

Specific weight represents the weight per unit volume and is calculated by multiplying density by gravitational acceleration. This property indicates how weight varies with volume and helps estimate the force exerted by the fluid on pipeline walls, which is essential for maintaining structural integrity.

Specific gravity is a dimensionless measure that compares a fluid's density to that of water at a particular temperature. This comparison aids in material selection, ensuring compatibility and efficiency. For example, specific gravity helps choose the optimal fluid for a system by comparing different options without unit constraints.

Compressibility describes how a fluid's volume changes under pressure. The bulk modulus quantifies this property, relating pressure change to volume change. Liquids have low compressibility, meaning their volume changes minimally under pressure. In contrast, gases are highly compressible, showing significant volume changes with pressure variations. This property is crucial in hydraulic systems, where understanding how much a fluid can be compressed under pressure ensures efficiency and safety.