disclosed is a combination of an infusion set consisting of infusion tubing and needle connected to a drug cartridge , the latter comprising a cylindrical body of inner diameter and having an open rear end , wherein a stopper is slidably disposed within the body . in an initial state after the cartridge has been filled , the stopper is partially disposed within the body such that the stopper protrudes from the rear end of the body by a distance . pushing the stopper into the cartridge by said distance , i . e . until the stopper is flush with the rear end , displaces a predetermined volume according to e , b and the cylindrical geometry of the cartridge , which is large enough to effect complete air priming of the cartridge , infusion tubing and infusion needle .