Spirometer with double rolling seals

A spirometer for measuring breathing functions of human subjects, said spirometer having a cylindrical casing with rear and side walls defining a cylindrical chamber open at one end;

• a cylindrical piston supported for axial movement within said chamber;
• said piston being connected to indicating means for indicating the axial travel of said piston;
• port means in said casing for the entry and removal of expired air into and from said chamber behind said piston;
• and sealing means disposed between said piston and the inner surface of said casing;
• wherein the improvement comprises
• said sealing means being in the form of first and second annular membranes each formed of elastomeric material;
• said first membrane being reversely folded upon itself to provide an outer annular end portion sealingly secured to the inner side wall of said casing and an inner annular end portion sealingly secured to said piston adjacent the rear end thereof;
• said second membrane also being reversely folded upon itself to provide an outer annular end portion sealingly secured to the inner side wall of said casing and an inner annular end portion sealingly secured to said piston adjacent the front end thereof;
• said first and second membranes being arranged with their respective folds facing each other;
• said first membrane having the major portion of its axial length extending rearwardly beyond said piston when said piston is at the forward limit of its stroke and said second membrane having the major portion of its axial length extending forwardly beyond said piston when said piston is at the rearward limit of its stroke;
• and spring means engaging said piston and exerting forces urging said piston rearwardly within said chamber, the forces exerted by said spring means being greater when said piston is at the rearward limit of its stroke than at the forward limit thereof.