The cross section of the aperture depends non-linearly on the control path of the valve member. The valve member is formed as a roll-off body (9), positioned opposite to the conduit aperture (7), and having a sealing surface, engaging on a counter surface (8), which contains the aperture. The roll-off body has an associated lever (17), for its movement on the counter surface. The sealing surface is contained in a sealing strip (10) of elastically deformable material. The strip is fastened to the counter surface, and to a segment (11) of the roll-off body. The segment extends above the strip, parallel to it, over the conduit aperture and is arched.
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