the pumping arrangements 3 and 4 include casings or shells 41 and 42 which are secured in a conventional manner which is not specifically shown in the drawing such as by screws or similar connectors to the control arrangement 5 and which are respectively connected by means of annular clamping elements 43 and 44 to the flanges 39 and 40 of the lateral portions 31 and 32 of the frame 2 hence the shells 41 and 42 define with the associated lateral portions 31 and 32 respective internal spaces each of these internal spaces is subdivided by a respective movable wall 45 and 46 into the aforementioned pumping chamber 31 or 32 and an actuating chamber 47 or 48 the respective movable wall 45 or 46 includes a respective flexible diaphragm 49 or 50 which is sealingly clamped at its outer periphery between the flange 39 or 40 and the shell 41 or 42 by the action of the respective clamping element 43 or 44 at its central region each of the diaphragms 49 or 50 is sealingly secured to a common shaft 51 that passes through the interior of the control arrangement 5 and interconnects the two movable walls 45 and 46 for movement in unison to mount the respective diaphragms 49 and 50 on the common shaft 51 there are provided two mounting plates 52 and 53 or 54 and 55 on the common shaft 51 which confine the central region of the respective diaphragm 49 or 50 between themselves the mounting plates 52 and 53 or 54 and 55 are mounted on the common shaft 51 by being confined between a respective shoulder 56 or 58 and a threaded fastener 57 or 59 such as a nut thus the mounting plates 52 and 53.