Pumps, reciprocating, flow variation

Dual-head reciprocating pumps offer lower solvent pulsation at the expense of mechanical complexity. Here two pistons fill and pump 180° out of phase and in theory provide pulseless flow. Dual-head pumps are more expensive and have either two check valves (series heads) or four check valves (parallel heads). Both designs generally provide for some type of pressure or flow feedback control to further compensate for minor flow variations during switching from one head to the next. A detailed discussion of the mechanics of these and other pumping systems is provided elsewhere (13). 

Reciprocating-piston pumps deliver a constant flow at a fixed backpressure. At high pressures some minor flow variations may arise due to the compressibility of the mobile phase. Some instruments incorporate a flow controller which provides a fixed backpressure for the pump to work against, independent of the column backpressure. The influence of pressure fluctuations, solvent compressibility, and solvent viscosity on the volumetric output of the pump is thereby eliminated. Reciproeating-piston pumps can provide continuous solvent delivery, fast solvent change over, gradient elution compatibility, and have low maintenance requirements. 

Figure 32.21 Reciprocating pump variation of flow rate with numbers of cylinders...

Compared to syringe type or reciprocating pumps, pneumatic amplifier pumps are very cheap. They tend to be rather difficult to dismantle for repairs, and some types are very noisy in operation. Because they do not provide a constant flow of mobile phase, they are not used much in analytical hplc. They can, however, operate at high pressures and flow rates and so are used mainly for packing columns, where high pressures are needed and variations in the flow rate through the column do not matter. 

The vast majority of pumps currently used in analytical HPLC are of the constant flow variety. With this type of pump, changes in the chromatographic system, for instance those leading to variations in the back pressure experienced by the pump, are compensated for, and the flow rate remains constant this is an important factor in most analytical applications. Two major types of constant flow pump exist the reciprocating piston design and the syringe type. 

The problem of pulses is solved by means of damping devices that consist of a hydraulic resistor and a hydraulic capacitor. The modern reciprocating pumping systems are equipped with two or more pistons, their motion being mechanically or electronically controlled in such way that both short and long term flow rate variations are substantially reduced. 

Reducing the non-shot Category III noise sources of the fixed wavelength detector will require attention to Improved thermomechan-Ical detector design to reduce noise due to thermal variations Imposed on the mobile phase/flow cell and on the sample and reference photodiodes, and continued reduction In HPLC reciprocating pump flow pulsations. 

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