16-10 Figure 16-15.  A passenger service unit (psu) is hinged over each row  of seats in an airliner. Four yellow continuous flow oxygen masks are  shown deployed. They are normally stored behind a separate hinged  panel that opens to allow the masks to fall from the PSU for use. Figure 16-16.  The crew can deploy passenger emergency continuous- flow oxygen masks and supply with a switch in the cockpit. The diluter-demand type regulator holds back the flow of  oxygen until the user inhales with a demand-type oxygen  mask. The regulator dilutes the pure oxygen supply with cabin  air each time a breath is drawn. With its control toggle switch  set to normal, the amount of dilution depends on the cabin  altitude. As altitude increases, an aneroid allows more oxygen  and less cabin air to be delivered to the user by adjusting  flows through a metering valve. At approximately 34,000  In a demand-flow oxygen system, the system pressure- reducing valve is sometimes called a pressure regulator.  This device lowers the oxygen pressure from the storage  cylinder(s) to roughly 60–85 psi and delivers it to individual  regulators dedicated for each user. A pressure reduction also  occurs at the inlet of the individual regulator by limiting the  size of the inlet orifice. There are two types of individual  regulators: the diluter-demand type and the pressure-demand  type.  [Figure 16-20] Demand-flow systems are similar to continuous-flow systems  in that a cylinder delivers oxygen through a valve when  opened. The tank pressure gauge, filter(s), pressure relief  valve, and any plumbing installed to refill the cylinder while  installed on the aircraft are all similar to those in a continuous- flow system. The high-pressure oxygen also passes through  a pressure reducer and a regulator to adjust the pressure and  flow to the user. But, demand-flow oxygen regulators differ  significantly from continuous-flow oxygen regulators. They  work in conjunction with close-fitting demand-type masks  to control the flow of oxygen.  [Figure 16-19] When oxygen is delivered only as the user inhales, or on  demand, it is known as a demand-flow system. During  the hold and exhalation periods of breathing, the oxygen  supply is stopped. Thus, the duration of the oxygen supply  is prolonged as none is wasted. Demand-flow systems are  used most frequently by the crew on high performance and  air transport category aircraft.  [Figure 16-18] Continuous-flow oxygen masks are simple devices made  to direct flow to the nose and mouth of the wearer. They  fit snugly but are not air tight. Vent holes allow cabin air  to mix with the oxygen and provide escape for exhalation.  In a rebreather mask, the vents allow the exhaled mixture  that is not trapped in the rebreather bag to escape. This is  appropriate, because this is the air-oxygen mixture that has  been in the lungs the longest, so it has less recoverable oxygen  to be breathed again.  [Figure 16-17] the passenger service unit (PSU).  [Figure 16-15]  Deployment  of the emergency continuous-flow passenger oxygen masks  may also be controlled by the crew.  [Figure 16-16] Demand-Flow Systems