Arrangement for producing an electrical signal proportional to forced vital lung capacity as measured by a spirometer

In a spirometer including an expansible chamber (22) of the piston type there is provided an arrangement for producing an electrical signal which is related to the volume of the chamber (22), said arrangement comprising: an indicia strip (82) associated with the piston (14) of the chamber (22) and including two columns (76, 78) of alternating light and dark bars, the columns (76, 78) being offset from one another by one-half the thickness of a bar; and two photodetectors (72, 74) associated with a fixed member (70) of the spirometer and arranged to respectively observe the columns (76, 78), the photodetectors (72, 74) generating signals which are shifted 90 DEG from one another as the columns (76, 78) pass through their respective field of view and which are continuously sampled as digital words, the consecutive words being compared and the comparison result processed to produce said electrical signal.