SSI America 90-2581 REV B manual Theory Of Operation, 4.1.1Liquid System Flow Path, Pump Cycle

4.THEORY OF OPERATION

4.1Pump Mechanical Operation

4.1.1Liquid 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.

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