Agilent Technologies Infinity II SFC Binary 1260 User Manual page 14

1
Introduction
Overview of the Hydraulic Path
A servo-controlled variable reluctance motor drives the two ball-screw drives
in opposite directions. The gears for the ball-screw drives have different
circumferences (ratio 2:1) allowing the first piston to move at double the speed
of the second piston. The solvent enters the pump heads close to the bottom
limit and leaves it at its top. The outer diameter of the piston is smaller than
the inner diameter of the pump-head chamber allowing the solvent to fill the
gap in between. The first piston has a stroke volume in the range of 20 μL to
100 μL depending on the flow rate. The microprocessor controls all flow rates
in a range of 1 μL/min to 5 mL/min. The inlet of the first pumping unit is
connected to the active inlet valve which is processor-controlled opened or
closed allowing solvent to be drawn into the first pump unit.
When turned on, the pump runs through an initialization procedure to
determine the upper dead center of the first piston of both pump channels.
The first piston moves slowly upwards to the mechanical stop of the pump
head and from there it moves back a predetermined path length. The
controller stores this piston position in memory. After this initialization the
pump starts operation with the set parameters for the two pump channels.
The inlet valve is opened and the down moving piston draws solvent into the
first pump head. At the same time the second piston is moving upwards
delivering into the system. After a controller defined stroke length (depending
on the flow rate) the drive motors are stopped and the inlet valve is closed.
The motor direction is reversed and moves the first piston up until it reaches
the stored upper limit and at the same time moving the second piston
downwards.
Then the sequence starts again moving the pistons up and down between the
two limits. During the delivery stroke of the first piston the solvent in the
pump head is pressed through the outlet valve into the second pumping unit.
The second piston draws in half of the volume displaced by the first piston and
the remaining half volume is directly delivered into the system. During the
drawing stroke of the first piston, the second piston delivers the drawn
volume into the system.
What is Solvent Compressibility Compensation?
Although the compressibility of liquids is orders of magnitude lower than the
compressibility of gases, without correction a noticeable volume error would
be seen if typical chromatographic solvents are compressed to operating
pressures as high as 600 bar. In addition, the compressibility depends on
pressure, temperature and the amount of dissolved gas. In order to minimize
14 1260 SFC Binary Pump