What does a variable area flow meter do?

• A. Maintains a constant differential pressure and keeps the flow area fixed.
• B. Maintains a constant differential pressure and allows the flow area to change with flow rate.
• C. Changes flow rate by increasing temperature
• D. Uses a fixed flow area with changing pressure

Answer: (B)

A variable area flow meter works by letting the flow area change as the fluid moves, while keeping the pressure drop across the meter essentially constant. In a rotameter, fluid flows through a tapered tube and a float sits inside. As flow increases, the float is pushed upward until the forces on it balance; this rise increases the annular area available for flow. With a larger area but about the same pressure difference, more fluid can pass, so the flow rate changes with the float’s position. The float height then serves as a direct indicator of the current flow rate.