The Continuity Equation for an incompressible fluid states: $A_1{v}_1=A_2{v}_2=\text{constant}$

• $Av$ is the volume flow rate (volume of fluid passing per unit time).
• Since the fluid is incompressible (constant $\rho$), preserving the volume flow rate is equivalent to preserving the mass flow rate ($\rho Av$).
• Therefore, the continuity equation is a statement of the Conservation of Mass: fluid mass entering a pipe section must equal the mass leaving it.