Pneumatic pump:

These types of pumps are simple, inexpensive and pulse free but suffer from limited capacity and pressure output. In this case, mobile phase is contained in a collapsible container housed in a vessel that can be pressurized by a compressed gas. These are not amenable to gradient elution and are limited to pressures less than 2000 psi.

Most commercial instruments are equipped with computer controlled devices for measuring the flow rate by determining the pressure drop across a restrictor located at the pump outlet. Some difference in signal from a preset value is then used to decrease or increase the speed of the pump motor. Composition of solvents may be continuously varied or in a stepwise fashion.