Which type of pump relies on a vacuum to draw liquid into the chamber?

The positive displacement pump operates by trapping a fixed amount of liquid and forcing that trapped volume into the discharge pipe. This action can create a vacuum within the pump chamber, allowing atmospheric pressure to push the liquid into the pump. As the pump cycles, the mechanical action of moving parts, such as gears or a diaphragm, reduces pressure in the inlet and draws the liquid into the chamber.

In contrast, centrifugal pumps use kinetic energy produced by a rotating impeller to move liquid, relying on the characteristics of the liquid and the design of the pump rather than creating a vacuum. Diaphragm and gear pumps are types of positive displacement pumps, but the key point is that the basic function of positive displacement pumps as a category is to create a vacuum that enables liquid to flow into the chamber, making this the appropriate and correct answer for the question.