Optimization of Flow Reproducibility

To achieve the flow and pressure precision needed for the more demanding HPLC applications, a number of proprietary electropneumatic pressure and flow control loops have been created to ensure repeatable pump performance. 

Pressure loops are used to control the pump delivery rate, by the placement of a sensitive and highly responsive pressure transducer downstream of the pump heads. Once a normalized baseline is established, the sensing circuitry regulates motor speed to keep the pressure constant. A momentary decrease in pressure will cause the control loop to accelerate the motor in order to increase the pressure. Although the pressure loop is effective at maintaining a smooth pressure delivery, it is done at the expense of flow accuracy. 

Flow control loops are designed to maintain constant retention times by providing constant flow delivery from the pump, commonly by the use of an encoder wheel attached to the main motor drive. The encoder accurately measures the rotation of the motor extremely accurately to parts-of-a-degree of rotation, and it regulates the motor revolutions to as constant a value as possible. In a well-maintained pumping system, a constant motor speed will deliver a constant flow for a given solvent. In modem pumps a flow control loop is often combined with a pressure loop, to provide both 

Chapter 3 Instrumentation for High-Performance Liquid Chromatography 

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