If we want to measure mass flow rate in a very simple way, we might cut open an artery and catch blood in a beaker for a specific length of time. After weighing the blood in the beaker to determine the mass, m, we would divide by the time required to collect the blood, t, to get to flow rate, Q = m/t. However, this is not always a practical method of measuring flow rate.

Instead, consider that for any control volume, conservation of mass guarantees that the mass entering that control volume in a specific time is equal to the mass leaving that same control volume over the same time interval plus the increase in mass inside the control volume.

mass mass

\ time/in V time/out

The continuity equation represents conservation of mass in a slightly different form, for cases where the mass inside the control volume does not increase or decrease and it is written:

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