Abstract: A cartridge (3) carrying an impeller (4) is inserted into a flowmeter sensor (1). The water pressure and shape tolerances as well as forces and bending moments acting on the system from the outside shall not affect the rotary behavior of the impeller (4). The outer surface (10) of the cartridge (3) is spaced apart from the inner surface (11) of the water space (20). A flange (14) is provided to seal the cartridge (3) against the water space (20). The cartridge (3) is supported linearly or in a punctiform manner in a guide area (32) of the inner surface (11).