Agilent Technologies 1100 Series, Service Manual

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198 1100 Series Nano Pump Service Manual
5 Introduction to the Nano Pump
Each pump head consists of two identical chambers, pistons and seals, plus an
active inlet valve and an outlet ball valve. The solvent volume in each chamber
is displaced by its piston. The pistons are directly moved by the reciprocating
ball-screw drives of the metering drive assembly. Due to the gear design of the
metering drive assembly, the pistons move in opposite directions, with piston
1 constantly moving at twice the speed of piston 2. The outer diameter of the
piston is smaller than the inner diameter of the chamber, allowing solvent to
flow in the gap between the piston and the chamber wall. The two chambers
are connected by the pressure dependent outlet ball valve.
The position of the solvent selection valve determines which of two solvents
will be sucked (low pressure) through the active inlet valve into chamber 1
during the intake stroke of piston 1. The active inlet valve is electrically
opened and closed, making its operation more precise at low pressures. The
stroke volume of piston 1 is between 2 µl and 100 µl, depending on flow rate.
When the nano pump is first turned on, the user is prompted to initialize the
pump. The initialization routine (occurring for both pump heads) first
determines the precise movement limits for both pistons.These limits are then
stored in the pump controller memory. Then, both pistons are set to their
default initial positions.
When pumping begins, the active inlet valve is opened and piston 1 begins its
intake stroke, sucking solvent into chamber 1. At the same time, piston 2
begins its delivery stroke, pumping (high pressure) the existing solvent in
chamber 2 out of the pump head. The pressure produced by piston 2 also
closes the outlet ball valve, preventing any chamber 2 solvent from
back-streaming into chamber 1. After a predefined piston 1 stroke length, the
servo motor is stopped, and the active inlet valve is closed. The pistons now
reverse directions. Piston 1 begins its delivery stroke (high pressure), and
piston 2 begins its intake stroke. Piston 2 is moving at only half the speed of
piston 1. The outlet ball valve is forced open by the pressure generated by
piston 1. Piston 1 begins to deliver the volume previously sucked into chamber
1. Because of the 2:1 speed ratio of the pistons, half of the solvent flow from
chamber 1 is forced out of the pump head, continuing into the pump hydraulic
path. The other half of the flow from chamber 1 simultaneously refills
chamber 2.
When piston 1 has completed its delivery stroke, the pistons reverse direction,
and the cycle is repeated.