In low-pressure systems, mixing is accomplished prior to the pump, at its low-pressure side and the overall flow rate is controlled by a single pump. Blending of the mobile-phase components is accomplished with controls which are calibrated in somewhat the same manner as those used for high-pressure mixers - that is, controlling the percentage of the strong component - though some systems allow the mixing of three liquids. In general, two types of blending systems are used.
In the first case, proportioning valves, normally solenoid operated, are used to deliver the individual liquids. The controller simply divides the signal according to the percentage of each component and each valve is opened for the proper period of time. Usually the valves deliver the individual liquids into a mixing chamber which then feeds the blend to the pump. In some systems, the mixing chamber may be missing and the valves feed the mobile-phase components through a mixing connector directly to the high-pressure pump.
The primary advantage of such systems is that they offer the ability to blend two or more liquids without the increased expense and maintenance of additional pump(s).
This is now the most popular scheme of gradient formation, although the precision of
eluent composition for medium concentration range is better for the high-pressure mixing.