A positive displacement device consisting of a piston and cylinder arrangement. The piston is driven by a crankshaft though a connecting rod such that the piston enters and leaves the cylinder expelling and drawing in liquid for transportation in a cyclic fashion. A tight tolerance is required between the piston and cylinder to ensure no leakage. Check valves are used to ensure that the flow is in the desired direction. Multiple cylinders operated out of phase are used to provide a more continuous flow. Variations include the plunger pump and the diaphragm pump.