Figure: A pressure trace as a diagnostic aid. (a) A properly performing solvent delivery system, (b) A pressure dip probably due to a bubble being "pumped." (c) An improperly "pulsing" solvent delivery system which probably has bad check valves. Note that the relative pulsation for a "good" and "bad" solvent delivery system must be determined for each system. Each solvent delivery system will be somewhat different.

Reference:

B.A. Bidlingmeyer, Practical HPLC Methodolgy and Applications, John Wiley & Sons, New Yok, NI, 1993, p78-80