Displacement meters measure flow by registering the number of fills and empties of the meter chamber. Which statement best describes this type of meter?

• A. They measure flow by pressure differential
• B. They measure flow by turbine blades
• C. They measure flow by filling and emptying cycles
• D. They measure flow by ultrasonic scattering

Answer: (C)

Displacement meters measure flow by counting the volume moved in each fill-and-empty cycle. In positive displacement meters, the chamber fills with fluid, the chamber is isolated and the fluid is moved out, and the cycle is completed. Each cycle transfers a fixed, known volume, so the flow rate is determined by how many cycles occur per unit time (and the total volume by summing cycles over time). This approach differs from pressure-differential meters (which infer flow from a pressure drop), turbine meters (which rely on spinning blades driven by flow), and ultrasonic meters (which use sound waves to gauge flow).