HPLC Products

Fluctuating Back Pressure is almost always caused by air in the pump.

The pump motor runs at a very carefully controlled speed to give a very precise solvent delivery rate. However if an air bubble enters the pump, for part of the piston's travel it does nothing more than compress the air bubble. During this time, eluent is still dripping out of the system waste pipe, and the back pressure is dropping. Then once the air bubble is compressed, the pressure in the piston chamber rises very rapidly. Once it exceeds the system back pressure, the outlet check valve opens and solvent flows again, causing the system back pressure to rise.

To eliminate an air bubble:

Stop the pump. Open the purge valve. Turn on the Purge setting, to make the pump run at maximum flow. The eluent will now flow out of the purge valve and may need to be collected in a beaker. Some pumps have a luer port for the attachment of a syringe, and in this case, the removal of the air may be facilitated by using the syringe to suck it out. Some pumps have some tubing to carry the eluent from the purge valve directly to waste. Whichever configuration you have, it would be helpful if you could see if an air bubble was removed. Once the air has been eliminated, stop the pump and close the Purge Valve. Then continue pumping normally, and observe the pressure guage. If this procedure has been successful, the pressure should be back to nomal, and the fluctuations should have stopped.