Since infants cannot cooperate, the apparatus shown in figure 1 was devised to determine their vital capacity. It consists of a 5 gallon (22.7 liter) closed container, similar to a milk can, of sufficiently heavy gage metal so that the walls are rigid. A water manometer is connected to measure the pressure within the container, which is connected by heavy walled rubber tubing with a Marriott face mask.1
The container is calibrated by pouring measured amounts of water into it. As shown in figure 2, the pressure increases 1 mm. for each 4.8 cc. of water added to the container, or 0.23 mm. per cubic centimeter.
The vital capacity of an infant is measured by placing the face mask quickly and tightly over his mouth and nose at the end of a full inspiration. The maximum of several determinations is taken as the vital capacity. If the infant draws
SMITH HH, McLANAHAN S, DAVISON WC. AN APPARATUS FOR DETERMINATION OF VITAL CAPACITY OF INFANTS. Am J Dis Child. 1942;63(1):92-93. doi:10.1001/archpedi.1942.02010010093008