Mask/regulator (first observer’s station) System shutoff valve Crew oxygen cylinder located in  forward cargo compartment Mask/regulator (second observer’s station) Mask storage box with mask/regulator (captain’s station) Mask storage box with mask/ regulator (first officer’s station) Emergency metering control Demand valve Air inlet check valve Pressure reducing valve Diluter control closing mechanism Air metering port lever Diaphragm Aneroid Oxygen metering port Inhale 16-12 Figure 16-19.  A demand regulator and demand-type mask work together to control flow and conserve oxygen. Demand-flow masks are  close fitting so that when the user inhales, low pressure is created in the regulator, which allows oxygen to flow. Exhaled air escapes  through ports in the mask, and the regulator ceases the flow of oxygen until the next inhalation. Figure 16-18.  Location of demand-flow oxygen components on a transport category aircraft. 