Which of these is a displacement meter, which measures flow by registering the number of times the meter chamber is filled and emptied?

• A. Electromagnetic Meters
• B. Ultrasonic Meters
• C. Piston and Nutating Disc Meters
• D. Turbine Meters

Answer: (C)

Displacement meters measure flow by counting how many times a fixed-volume chamber is filled and emptied. A piston type traps a precise amount of water in its chamber and moves to push that volume out with each cycle, so every cycle represents a known quantity of water. Nutating disc meters work the same way: a wobbling disc creates sealed, fixed-volume spaces as water flows, and each completion of a fill-and-empty cycle corresponds to a set amount of volume. Because the measurement is based on counting these discrete, fixed volumes, the total flow is the cycle count times the chamber volume.

Other methods measure flow differently. Electromagnetic meters detect flow velocity using a magnetic field and the induced voltage; ultrasonic meters assess flow by timing or Doppler methods related to velocity; turbine meters relate the speed of a rotating turbine to flow. These are velocity-type meters, not displacement meters, which is why the piston and nutating disc combination is the correct choice.