# Formula for pressure ripple

Can someone provide a reference to the mathematical formula we use for Pressure Ripple on an HPLC?  In particular I'm wondering about the meaning of negative pressure ripple, which I have seen a couple times now on our HPLCs; I thought that pressure ripple looked at the absolute value of the change of pressure, which makes the meaning of negative ripple a little murky.  Ordinarily we look for pressure ripple below a certain threshold; if the value is negative I presume we would just take its absolute value?

• Hello,

The pressure ripple is the pressure fluctuation or amplitude of pressure deflection up (positive) or down (negative) and that is divided by the total system pressure x 100 to give % ripple.  So, if the total pressure is 200 bar and your pressure is fluctuating by 10 bar then your ripple will be 5%.  Assuming you have perfectly functioning pump (no leaks, no air in the pump head and check valves are operating well), you can still have a high pressure ripple due to incorrect solvent compressibility setting.  Water for example has a compressibility of 46 so if it was set too high or too low, the ripple will not be close to zero as expected.  This is where you will get negative values.  When you do, then increase the compressibility value.  If positive, decrease it.  Pure solvents are easy to get as there are tables but when you start mixing different solvents that's the only  way to determine an appropriate value.

I hope that helps.