Device for controlling a piston pump unit for liquid chromatography

A method to control a piston pump for liquid chromatography, the piston pump including at least two phase-offset cyclically working piston-cylinder units, which produce a predefined flow of a liquid medium to be delivered at an outlet port, wherein a constant system pressure results at the outlet port, the method comprising:

• in a pre-compression phase of one cycle of at least one of the piston-cylinder units, compressing with a piston in the piston-cylinder unit;
• measuring a first pressure at a first time and a second pressure at a second time during the pre- compression phase in which an adiabatic heating effect is negligible in the piston-cylinder unit;
• extrapolating the first pressure at the first time and the second pressure at the second time to determine a calculated time in which a pressure of the piston-cylinder unit will be at the constant system pressure;
• determining a detected time in which the pressure of the piston-cylinder unit is measured with a pressure sensor to be at the constant system pressure;
• calculating a time difference based on the calculated time and the detected time;
• and
• calculating a first correction amplitude of the piston speed for at least one piston-cylinder unit based on the time difference,
• in which a piston speed is constant during the pre-compression phase,
• in which the calculated first correction amplitude of the piston speed is based on a decreasing exponential function, and
• applying a first corrected piston speed to at least one piston-cylinder unit during the delivery phase, in which the delivery phase is subsequent to the pre-compression phase, the first corrected piston speed is based on the calculated first correction amplitude of the piston speed.