30 the bottom wall 15 of which is made of a material which allows electromagnetic waves to pass through it such as plastic or glass the upper wall 31 of the chamber 30 extends parallel to the lower wall 15 and is slidably mounted for movement perpendicular to its plane on a plurality of posts 32 extending perpendicularly from the bottom wall 15 the slidable upper wall 31 is biased towards the impeller 14 by helical coil springs 33 mounted on the posts 32 helical coil springs 33 are fixed adjacent the top end of each post 32 by element 52 end stops 34 are provided on the posts 32 for limiting the travel of the slidable upper wall 31 towards the impeller 14 an annular flexible diaphragm 35 extends around the impeller 14 between the upper and lower side walls 31 15 to form the side wall of the chamber the impeller 14 is mounted in the chamber 30 with the center thereof in registration with an inlet duct 36 extending from the slidable upper wall 31 an outlet duct 37 extends from the slidable upper wall 31 adjacent the radially outermost portion of the impeller 14 in use the apparatus functions exactly as before except the upper wall 31 of.