A system for inflating a balloon (8) of a balloon catheter (4) with a liquid inflating medium which removes air during inflation of the balloon (8) comprises a vertically mounted syringe (36) containing the liquid inflating medium for inflating the balloon (8). Air in the system is displaced into a return lumen (30) and a return line (46). Air drawn into the syringe (36) with the liquid inflating medium is trapped between a piston (39) of the syringe and a top surface of the liquid inflating medium in the syringe, and thereby separated from the liquid inflating medium. Re-inflating and deflating of the balloon continues until the level of air in the balloon, the flow and return lumens and the flow and return lines (44,45) has been reduced to a predefined level. Then, an accurate determination of the inflation pressure can be read from first and second pressure sensors (50,51 52,53) on the return and flow lines, respectively. The balloon catheter may be used to determine the volume and transverse cross-sectional area of a lumen or vessel within which the balloon is inflated.
