Optical digitizer for measuring spirometer output

In a spirometer having a casing, piston means within said casing constrained to rectilinear motion, and means forming an air tight seal between said piston means and said casing, said casing, piston means and seal means together forming a substantially air tight expansible chamber, said casing having an opening through which expiratory flow is introduced into said expansible chamber, the introduction of expiratory flow producing movement of said piston means, an improvement comprising:

• optically detectable indicia fixed with respect to one of said piston means and said casing;
• first and second photodetector means, fixed with respect to the other of said piston means and said casing for generating output signals in response to relative motion of said optically detectable indicia with respect to said first and second photodetector means, said first photodetector means being arranged to observe a first field of view and said second photodetector means being arranged to observe a second field of view, said indicia passing through said field of view as said piston means moves, said fields of view being arranged so that the output signals from said first photodetector means are out of phase with the output signals from said second photodetector means;
• means for converting said output signals to a repeating sequence of n-bit digital words;
• means for periodically combining one n-bit digital word with a previous n-bit digital word to form a 2 n-bit digital word;
• a counter;
• and
• means responsive to said 2 n-bit digital word for incrementing or decrementing said counter, said counter thereby generating an electrical signal related to the volume of said expansible chamber.