An apparatus for inserting a venturi tube having an inlet, an aspirator inlet and an outlet directly within a fluid flow stream, with the aspirator inlet not in contact with the fluid flow stream is disclosed. A plate having a first aperture leading to the venturi inlet, and a series of second apertures both supports the venturi within a conduit and divides the fluid flow stream into a first stream which flows through the venturi tube and a second stream which flows around the venturi tube. A variety of adjusting mechanisms are disclosed for varying the cross sectional area of the second apertures relative to the cross sectional area of the venturi tube. The apparatus is adapted to allow the back pressure within the venturi tube (which increases as the fluid flow rate increases) to force an increasing proportion of the flow stream to flow around the venturi tube in order to maintain a relatively constant aspiration rate within the venturi tube. The adjustment mechanisms disclosed include a second plate, similar to the first plate, which is manually rotated and a blocking ring which moves responsive to changes in rate of flow of the fluid flow stream.