From: Meneely GR, Kaltreider NL.  The volume of the lung determined by helium dilution. Description of the method and comparison with other methords.  Journal of Clinical Investigaton, 1949 28(1): 129-139, page 130.

“The spirometer circuit is diagrammed in Figure 1.  The seven-liter cylindrical spirometer with obliterated internal dead space write the respiratory tracing on a kymograph drum. The volume of the spirometer may be read from the scale and pointer and, by transfer, from the respiratory tracing itself. The subject is connected to the circuit through a rubber mouthpiece on a three-way valve. The expired air passes vertically through a soda-lime canister for the most efficient absorption of carbon dioxide. Oxygen may be added at any desired rate through the diaphragm type flow control and meter on a tank of “medical” oxygen. A blower impels the expired air through the water bottle which contains many glass bead to break up the bubbles, and prevent disagreeable bumping due to large bubbles. Provision is made for the introduction of helium at the spirometer outlet. Part of the return flow shunts through the analytical cell and returns via the cell outlet to rejoin the return stream. The pressure differences in different segments of the circuit are small: slight negative pressure obtains between the soda-lime and blower, and positive pressure elsewhere. The soda-lime and the water bottle isolate the the blower to a degree, preventing vibration of the air column at the mouthpiece.”