Diaphragm Pump In a diaphragm pump, a diaphragm is hydraulically moved backwards and forwards rapidly by a simple piston pump. The cavity on the other side of the diaphragm is connected to two non-return valves operating in opposition. When the diaphragm moves back, solvent is drawn through one non-return valve into the cavity from the solvent reservoir and when the diaphragm moves forward the solvent is expelled through the other non-return valve to the column. The driving piston is situated on the other side of the diaphragm and uses a separate solvent supply. Consequently, the piston never comes into contact with the actual mobile phase. The movement of the diaphragm is relatively small and, thus, can be operated at high frequencies and any pump noise can be electronically removed. This reduces the high frequency noise in the detector not does not completely eliminate it.
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Basic-HPLC Pump Diaphragm
The Diaphragm Pump The unique property of the reciprocating diaphragm pump is that the actuating piston does not come into direct contact with the mobile phase and thus, the demands on the piston-cylinder seal are not so great. The diaphragm has a relatively high surface area and thus, the movement of the diaphragm is relatively small and consequently the pump can be operated at a fairly high frequency. High frequency pumping results in a very significant reduction in pulse amplitude and, in addition, high frequency pulses are more readily damped by the column system. Nevertheless, it must be emphasized that diaphragm pumps are not pulseless. A diagram of a diaphragm pump, showing its mode of action is depicted in figure 10 Figure 12. The Action of a Diaphragm Pump
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Basic-HPLC Sample-Valve
The wheel driving the crank rotates in an anti-clockwise direction and in position (1) the diaphragm has been withdrawn causing the pumping cavity behind the diaphragm to be filled with solvent. In position (2), the piston advances and when it passes the pumping fluid inlet, it starts compressing the diaphragm expressing solvent to the column. In position (3) the diaphragm has been compressed to its limit and the piston starts to return. In position (4) the piston moves back withdrawing the diaphragm sucking liquid into the pumping cavity ready for the next thrust. The inlet from the solvent supply and the outlet to the column are fitted with non-return valves in the usual manner. Due to the large volume of the pumping cavity, any gradient profile would be seriously distorted so this type of pump is not often used for analytical purposes but is often
