A diaphragm water pump is a positive displacement pump that uses a combination of reciprocating action and either a flapper valve or a ball valve to transfer liquids. This pump is sometimes referred to as a membrane pump. Diaphragm pumps are self priming and are ideal for viscous liquids.

There are three main types of diaphragm pumps:

• Those in which the diaphragm is sealed with one side in the fluid to be pumped, and the other in air or hydraulic fluid. The diaphragm is flexed, causing the volume of the pump chamber to increase and decrease. A pair of non-return check valves prevent reverse flow of the fluid.

• Those employing volumetric positive displacement where the prime mover of the diaphragm is electro-mechanical, working through a crank or geared motor drive, or purely mechanical, such as with a lever or handle. This method flexes the diaphragm through simple mechanical action, and one side of the diaphragm is open to air.

• Those employing one or more unsealed diaphragms with the fluid to be pumped on both sides. The diaphragm(s) again are flexed, causing the volume to change.