the membrane the front view of fig2 illustrates a pressure sensor 44 which may be used with the independent pressure sensor cavity 15 of the cartridge 2 or with the pressure sensor cavity 36 of the first hub chamber 7 the ply 40 makes the pressure sensor less sensitive to the elasticity of the membrane 13 in the sensor pressure area in addition the shape of the cavity 15 shall be made such that air can be eliminated easily when fluid is passing into the cavity e g by having a round shaped bottom of the cavity within the direction of the flow in the embodiments discussed previously each port has a dedicated valve this is not the case for the pump inlet and the pump outlet which are always kept open the invention encompasses several other features not necessarily illustrated on the figures for instance the cycler or the cartridge pumping element assembly may contain a window for detecting correct positioning of the flexible tube of the pump as shown in fig2 circle when the system functions the pressure is preferably always maintained positive with respect to the drain this is a safety measure which avoids said contaminated liquid to potentially infect the patient advantageously the liquid pressure entering and exiting the cartridge is sensed and if necessary the.