Rotary potentiometer on wheels. Large, heavy, messy, poor frequency response. Volume shows a steady reduction during breathing. Rate indicates oxygen absorption. Addition of O 2 replenishment allows continuous breathing Dry spirometers Various attempts to eliminate the water, usually bellows.
Problem is to get high compliance combined with accurate measure of volume.
Plastic bag is high compliance but poor measurement. Piston is accurate measurement but high stiffness. Non-linear movements are more complex to counterbalance. Measure flow as pressure drop across a flow resistance. Normal flow rates and hence pressures are well below this.