What is a Variable Area Flow Meter? What is an example of one?

• A. A device that measures humidity
• B. An impulse line
• C. A heat exchanger
• D. A flow meter that maintains a constant differential pressure and increases flow area with an increase in flow rate

Answer: (D)

Variable area flow meters regulate flow by changing the size of the flow path as the fluid moves. In a classic example, a float sits inside a tapered tube; as flow increases, the float is pushed upward. Moving up the tube increases the annular area around the float, allowing more fluid to pass. The float finds a position where the upward forces (buoyancy and drag) balance the weight of the float. Because the tube is narrow at the bottom and widens toward the top, higher float positions correspond to higher flow rates, so you read the flow directly from the float height on the scale. A common, real-world example is the rotameter.