If you are you are using rotary valves in your HPLC system, there will always be a short time whilst valve switching in which the flow is “in-between” valve positions. The valve drive VU4.1 is developed to minimize this phenomenon by its fast switching times to prevent back-pressure build up. Additionally KNAUER offers a number valves as break-free version.
However, some valves like the multiinjection-valves have longer switching times due to their inherent geometry. When working with these valves, keep in mind that high flowrates* can result in back pressure peaks during the switching process.
If pressure peaks are observed, you can reduce the flowrate during the injection process and then increase the flowrate after the valve switched to the desired inject position.
*the exact flowrate upon which backpressure peaks can occur depends on used solvents and capillary ID.
