Constant pressure pump:

In this type of pump, pressure delivered through a large piston drives the mobile phase. Since the pressure on the solvent is proportional to the ratio of the area of the two pistons, usually between 30:1 and 50:1, a low pressure gas source of 1-10 atm can be used to generate high liquid pressures (1-400 atm). A valving arrangement permits the rapid refill of the solvent chamber whose capacity is about 70 mL. This system gives pulse less and continuous pumping, involving high flow rates for preparative applications. This type of pump is useful for pumping columns but inconvenient for solvent gradient columns.