Which describes mass flow vs volumetric flow?

• A. Mass flow is volume per time
• B. Volumetric flow is mass per time
• C. Mass flow is actual mass per unit time
• D. Volumetric flow is not used for metering

Answer: (C)

Mass flow rate is the amount of matter passing a point per unit time, i.e., the actual mass transported each second (commonly kilograms per second). In contrast, volumetric flow rate is how much volume moves per unit time (cubic meters per second or liters per minute). The two are connected by density: m_dot = rho * V_dot. That means a gas can have the same mass flow with different volume flow if its density changes with pressure and temperature. This is why fuel mass flow is often measured directly for accurate metering and combustion control, since mass reflects the actual amount of fuel entering the engine regardless of density. The statement that mass flow is the actual mass per unit time is the best description.