Figure 16-17.  Examples of different continuous-flow oxygen masks. 16-11 feet, the diluter-demand regulator meters 100 percent oxygen.  This should not be needed unless cabin pressurization fails.  Additionally, the user may select 100 percent oxygen delivery  at any time by positioning the oxygen selection lever on the  regulator. A built-in emergency switch also delivers 100  percent oxygen, but in a continuous flow as the demand  function is bypassed.  [Figure 16-21] Pressure-demand oxygen systems operate similarly to diluter- demand systems, except that oxygen is delivered through the  individual pressure regulator(s) under higher pressure. When  the demand valve is unseated, oxygen under pressure forces  its way into the lungs of the user. The demand function still  operates, extending the overall supply of oxygen beyond that  of a continuous-flow system. Dilution with cabin air also  occurs if cabin altitude is less than 34,000 feet. Permanently mounted EPD systems are also available. They  typically integrate with an electronic valve/regulator on  the oxygen cylinder and come with an emergency bypass  switch to provide continuous-flow oxygen should the system  malfunction. A liquid crystal display (LCD) monitor/control  panel displays numerous system operating parameters and  allows adjustments to the automatic settings. This type of  A recent development in general aviation oxygen systems is  the electronic pulse demand oxygen delivery system (EDS).  A small, portable EDS unit is made to connect between the  oxygen source and the mask in a continuous-flow oxygen  system. It delivers timed pulses of oxygen to the wearer on  demand, saving oxygen normally lost during the hold and  exhale segments of the breathing cycle. Advanced pressure  sensing and processing allows the unit to deliver oxygen  only when an inhalation starts. It can also sense differences  in users’ breathing cycles and physiologies and adjust the  flow of oxygen accordingly. A built-in pressure-sensing  device adjusts the amount of oxygen released as altitude  changes.  [Figure 16-24]  Different flow indicators are used to provide verification  that the oxygen system is functioning. Other demand-flow  indicators are built into the oxygen regulators.  [Figure 16-23] Flow indicators, or flow meters, are common in all oxygen  systems. They usually consist of a lightweight object, or  apparatus, that is moved by the oxygen stream. When  flow exists, this movement signals the user in some way.  [Figure 16-23]  Many flow meters in continuous-flow oxygen  systems also double as flow rate adjusters. Needle valves  fitted into the flow indicator housing can fine-adjust the  oxygen delivery rate. Demand-flow oxygen systems usually  have flow indicators built into the individual regulators  at each user station. Some contain a blinking device that  activates when the user inhales and oxygen is delivered.  Others move a colored pith object into a window. Regardless,  flow indicators provide a quick verification that an oxygen  system is functioning. Both diluter-demand and pressure-demand regulators also  come in mask-mounted versions. The operation is essentially  the same as that of panel-mounted regulators.  [Figure 16-22] Pressure-demand regulators are used on aircraft that regularly  fly at 40,000 feet and above. They are also found on many  airliners and high-performance aircraft that may not typically  fly that high. Forcing oxygen into the lungs under pressure  ensures saturation of the blood, regardless of altitude or  cabin altitude. Flow Indicators