Flow Control - Thermo Scientific Surveyor LC Pump Plus Hardware Manual

1
Introduction

Flow Control

Flow Control
12 Surveyor LC Pump Plus Hardware Manual
Figure 13 contains a schematic of the Surveyor LC Pump Plus flow control hardware.
A stepping motor that runs at 12 800 pulses per revolution controls the flow rate of the
mobile phase. The stepping motor drives a camshaft that is fitted with two cams. As the cams
rotate, the cam-followers at the back of the pistons translate the rotary motion of the cams
into linear motion. This linear motion moves the two parallel pistons back and forth inside
the pump head.
The eccentric shape of the cams determines the stroke length of the pistons. The stroke length
of the primary piston is 6 mm, and the stroke length of the secondary piston is 3 mm. Both
pistons are 1/8 in. in diameter, so their respective stroke lengths translate to a stroke volume of
48 μL for the primary piston and a stroke volume of 24 μL for the secondary piston.
The pump cycle (see Figure 14
consists of three regions. The first region, which is the solvent proportioning region, begins at
a 20° rotation with the start of the intake stroke for the primary piston. As the primary piston
begins to draw back, it creates a low-pressure zone that causes the inlet check valve to open. As
the inlet check valve opens, eluent enters the primary pump head chamber.
Solvent proportioning occurs during the intake stroke of the primary piston. The valves of the
solvent proportioning assembly open and close in the following order: B, C, D, and then A.
During the intake stroke, the outlet check valve is closed, and all flow to the system comes
from the secondary piston.
The second region, which is the compressibility region of the pump cycle, begins at 180° as
the primary piston starts its forward stroke. The resulting high-pressure zone causes the inlet
check valve to close while compressing the eluent in the primary pump chamber. As the
pressure of the eluent in the primary pump head exceeds the column pressure, the outlet
check valve opens and allows eluent to enter the secondary pump chamber. For about 40° of
the cam cycle, both pistons contribute flow to the system. The compressibility region of the
pump cycle ends at 260° of the cam cycle, when the secondary piston reaches full intake.
During the compressibility region of the cam cycle, which begins with the forward stroke of
the primary piston and ends with the intake stroke of the secondary piston, the on-board
CPU monitors the backpressure of the eluent, as well as the position of the pistons, and
adjusts the speed of the stepping motor to maintain constant flow.
The third region of the pump cycle extends from 260° to 20° rotation. In the third region of
the pump cycle, the primary piston is providing all of the flow to the system as well as refilling
the secondary piston chamber.
on page 14), which is one full rotation of the camshaft,
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