Which of the following is a positive displacement pump?

• A. Axial flow
• B. Diaphragm
• C. Centrifugal
• D. Magnet drive

Answer: (B)

A positive displacement pump moves a fixed amount of fluid with each operation by trapping a defined volume and then releasing it. A diaphragm pump does this with a flexible membrane that flexes to change the chamber volume: when the diaphragm moves outward, fluid is drawn in; when it moves inward, that same fixed amount of fluid is pushed out. That per-stroke, fixed-volume action is the telltale sign of positive displacement pumping.

In contrast, axial flow and centrifugal pumps rely on an impeller to impart energy to the fluid, creating flow and pressure through kinetic energy transfer. Their output depends on system head and speed, not a fixed volume per cycle, so they’re considered dynamic rather than positive displacement pumps. Magnet drive describes how the pump is driven (a drive method), not the fundamental pumping action, so it doesn’t define the pump as positive displacement.