Reciprocating piston pump:

It is the most popular type of pump as it is inexpensive and can permit a wide range of flow rates. It consists of a small motor driven piston moving rapidly back and forth in a hydraulic chamber that may vary from 35 to 400 µL. A piston sucks in solvent from the mobile phase reservoir through means of check valves. Commonly, a hydraulic fluid transmits the pumping action to the solvent via a flexible diaphragm; thus, minimizing solvent contamination and corrosion problems with pump parts.

A wide range of flow rates may be obtained by varying either the stroke volume during each cycle of the pump or the stroke frequency. Delivery of solvent through reciprocating pump is continuous without any restrictions on the reservoir or operating time. These have extremely small initial volume and accurate elution gradient. Its advantages involve small internal volume (35 to 400 µL), their high output pressures (up to 10,000 psi), their ready adaptability to gradient elution and their constant flow rates which are largely independent of column back pressure and solvent viscosity.