From: Solandt OM, Ridout JH.  The duration of the recovery period following strenuous muscular exercise.  Proceedings of the Royal Society of London, 1933: 113: 327-344, page 329.

“As shown in the diagram, the air expired by the subject is first collected in the 10-liter spirometer. From the spirometer it is pumped through a cooling coil, through the meter, and then into a Douglas bag.  In this way the air is metered before entering the bag, and the bag is only used to collect the air so that a sample may be taken.

“The valves used were of the conventional type, consisting of a circular rubber flap, seating on a metal ring. They were tested before each experiment. The pump used was of the movable blade rotary type and was driven by an electric motor. The meter used was a Sargent wet test gas meter. This type of meter is very sensitive to changes in water level, as Krogh (1929) has pointed out, hence the water level was checked before each experiment. The correct water level was determined by checking the meter against a small Bohr meter which has been calibrated by displacement of air. The accuracy of the Sargent meter was guaranteed by the makers to within 1 percent at rates of flow up to 50 cubic feet per hour.  During the experiments the rate of metering was never allowed to exceed this value. The cooling coil was used to prevent too great a rise in meter temperature owing to the passage of warm expired air through the meter. The effectiveness of this device is shown by the fact that the meter temperature seldom changed as much as one degree during an experiment (2 to 3 hours). By cooling the air, complete saturation with water vapor was also ensured.”