Im just curious about how is it potentially wrong if you got a person to breathe normally and at rest into the glass tube, I got my A grade buds and they said that was a viable option. As if you try to pull the rubber sheet slightly down and up uniformly, it would be impossible to know to what extent the volume of the tidal volume was accurate. If you had someone breathe into the glass tube and out, it would depict an accurate representation of the tidal volume, as its a similar basis for the mechanism of a spirometer, in that the balloon and chambre are decreasing and increasing with regards to the volume of air inside them.