User manual

Bio-inert Quaternary Pump User Manual 15

Introduction

Overview of the Hydraulic Path

When turned on, the pump runs through an initialization procedure to

determine the upper dead position of the first piston. The first piston moves

slowly upwards into the mechanical stop of the pump chamber and from there

it moves back for a defined distance. The controller stores this piston position

in memory. After this initialization the pump starts operation with the set

parameters. The active inlet valve opens and the down-moving piston draws

solvent into the first pump chamber. At the same time the second piston

moves upwards delivering to the system. After a controller-defined stroke

length that depends on the flow rate the drive motor is stopped and the active

inlet valve closes. The motor direction is reversed and moves the first piston

up until it reaches the stored upper limit and at the same time the second

piston moves downwards. Then the sequence starts again moving the pistons

up and down between the two limits. During the up movement of the first

piston the solvent in the pump chamber is pressed through the outlet valve

into the second pump chamber. The second piston draws in half of the volume

displaced by the first piston and the remaining half volume is directly

delivered to the system. During the drawing stroke of the first piston, the

second piston delivers the drawn volume to the system.

For solvent compositions from the solvent bottles A, B, C, D the controller

divides the length of the intake stroke in certain fractions in which the

gradient valve connects the specified solvent channel to the pump input.

For specifications of the quaternary pump, see “Performance

Specifications” on page 26.

How Does Compressibility Compensation Work?

The compressibility of the solvents in use will affect retention-time stability

when the back pressure in the system changes (for example, ageing of

column). In order to minimize this effect, the pump provides a compressibility

compensation feature which optimizes the flow stability according to the

solvent type. The compressibility compensation is set to a default value and

can be changed through the user interface.

Without a compressibility compensation the following will happen during a

stroke of the first piston. The pressure in the piston chamber increases and

the volume in the chamber will be compressed depending on back pressure

and solvent type. The volume displaced into the system will be reduced by the

compressed volume.