A spirometer has a housing with a wall having an opening therein. A bag made up of a first, stiff sheet and a second, flexible sheet having an exhaust slit therein as well as an inlet tube is supported within the housing with the first sheet lying against the wall and with the inlet tube extending through the opening. A valve flap within the bag overlies the inlet tube and a valve bar in the housing is spring urged with a selected force to press the bag, between the slit and the tube, against the wall. A primary plate is hinged to the housing at its top to lie against the second sheet. A secondary plate is similarly hinged and is power controlled to be spaced from or to press against the primary plate through an intervening cushion. The position of the primary plate is indicated mechanically and also by electrical instrumentation.