Figure 9-8 Flow tube constriction. The lower portion of the illustration represents the lower portion of two flow tubes. The clearance between the head of the float and the flow tube is narrow. The equivalent channel is tubular because its diameter is less than its length. Viscosity is dominant in determining gas flow rate through this tubular constriction. The upper portion of the illustration represents the upper portion of a flow tube. The equivalent channel is orificial because its length is less than its width. Density is dominant in determining gas flow rate through this orificial constriction. (Adapted from Macintosh R, Mushin WW, Epstein HG: Physics for the Anaesthetist, 3rd ed. Oxford, UK, Blackwell Scientific Publications, 1963.)