Method of forming a flow restrictor

A flow restrictor is made by flattening a tube of flexible material about a member of cross-sectional characteristics representative of a desired flow passage, and securing the tube there, e.g. by rf welding. The member is then removed leaving a determinate flow passage in which problems of creep do not arise. The restrictor may have parallel to the flow passage a portion of the tube which is not permanently secured together so as to leave a by-pass which can be open to give a comparatively unrestricted flow passage or can be temporarily completely closed by external compression. A single tube may have in line a plurality of such restrictors with by-passes, the flow passages of the various restrictors being of different flow characteristics. The flow characteristic required is obtained by closing off the by-pass associated with the selected flow passage.