Anti-siphon device for sanitary appliance

A backflow preventing device located downstream of a water control member of a sanitary fitting comprising a casing defining a cavity which communicates with a water feeding pipe and with an air inlet, as well as with two shutting members, serially mounted and displaceable under the effect of a depression in the water feeding pipe, thereby connecting the water feeding pipe with the open air through said cavity, said casing having an outer wall provided with passes establishing a communication between the inside of the casing and ambient air;

• a first umbrella-shaped membrane fixed to said outer wall by a central pin and resting against the inner side of said outer wall through its periphery, whereby in a rest state, the passage are closed;
• said casing further having an inner wall provided with bores establishing a communication between the inside of said casing and the water feeding pipe;
• a second thick membrane forced against the outer side of said inner wall through its central portion by resilient means having an adjustable force, whereby in a rest position, a peripheral portion of said second membrane closes said bores;
• a member fastened with, but spaced apart from, the inner wall of said casing and located inside said casing, said member having openings which provide communication between the passages of the outer wall and the bores of the inner wall when said first and second membranes are resiliently displaced by a depression in the water feeding pipe, said membrane limiting the extent of axial deformation of said first membrane to avoid any damage to said first membrane.