4-1
4. THEORY OF OPERATION
4.1
Pump Mechanical Operation
4.1.1 Liquid System Flow Path
The flow path of the Binary Solvent Delivery Module starts at the
inlet reservoir filter passes through the inlet check valve, then through
the pump head, and finally exits through the outlet check valve.
4.1.2 Pump
Cycle
The pump cycle consists of two phases, the pumping phase, when
fluid is metered out of the pump at high pressure, and the refill phase,
when fluid is rapidly drawn into the pump.
During the pumping phase, the pump piston moves forward at a
programmed speed; this results in a stable flow from the pump. The piston
is driven by a linear rapid refill cam which is belt driven by the motor.
At the end of the pumping phase, the pump enters the refill phase.
The piston quickly retracts, refilling the pump head with solvent, and
the piston begins to move forward again as the pumping phase begins.
The motor speed is increased during refill to reduce refill time and
to pre-compress the solvent at the beginning of the pumping phase.
Since the output flow completely stops during refill, an optional,
external pulse damper is necessary for applications requiring
extremely low pulsation levels.
For optimal operation of the check valves, a back-pressure of at least
25 psi is required. Operating at lower pressures can lead to improper
seating of the valves and subsequently inaccurate flow rates.
4.1.3 Pulse Damping
The diaphragm-type pulse damper (inside the pump drawer)
consists of a compressible fluid (isopropanol) held in an isolated
cavity by an inert but flexible diaphragm. During the pumping phase
of the pump cycle, the fluid pressure of the mobile phase displaces the
diaphragm, compressing the fluid in the cavity and storing energy.
During the pump refill phase the pressure on the diaphragm is reduced
and the compressed fluid expands, releasing the energy it has stored.
This helps to stabilize flow rate and pressure. The amount of mobile
phase in contact with the pulse damper is small, only 0.25 mL at 2,500
psi, and the geometry used insures that the flow path is completely
swept, so solvent “memory effects” are virtually eliminated.
To be effective, the pulse damper requires a backpressure of
approximately 500 psi or greater.
