The present invention relates to a method for calibrating and/or monitoring a flow sensor of a medical system, which flow sensor is fluidically connected and/or connectable to a compensating volume, said method having the steps of: adjusting or measuring a first pressure level； opening a fluid connection such that liquid flows through the flow sensor out of or into the compensating volume； using the flow sensor to measure a first liquid quantity V3 flowing through the flow sensor； adjusting or measuring a second pressure level； determining the first liquid quantity V3 on the basis of the first and second pressure level； and calibrating and/or monitoring the flow sensor on the basis of the first liquid quantity measured by means of the flow sensor and on the basis of the first liquid quantity determined using the first and second pressure level.
