BREATHING OXYGEN D C B A 16-15 Figure 16-25.  The key components of a built-in electronic pulse demand oxygen metering system: (A) electronic regulator, (B) oxygen  station distributer unit, (C) command/display unit, (D) emergency bypass switch.  Figure 16-24.  A portable two-person electronic pulse-demand  (EPD) oxygen regulating unit. Figure 16-26.  Color-coded tape used to identify oxygen tubing. Tubing-to-tubing fittings in oxygen systems are often  designed with straight threads to receive flared tube  connections. Tubing-to-component fittings usually have  straight threads on the tubing end and external pipe threads  (tapered) on the other end for attachment to the component.  The fittings are typically made of the same material as the  tubing (i.e., aluminum or steel). Flared and flareless fittings  are both used, depending on the system.  Five types of valves are commonly found in high-pressure  gaseous oxygen systems: filler, check, shutoff, pressure  reducer, and pressure relief. They function as they would in  any other system with one exception: oxygen system shutoff  valves are specifically designed to open slowly.  electronic metering of oxygen has also been developed for  passenger emergency oxygen use in airliners.  [Figure 16-25]  Tubing and fittings make up most of the oxygen system  plumbing and connect the various components. Most lines  are metal in permanent installations. High-pressure lines are  usually stainless steel. Tubing in the low-pressure parts of the  oxygen system is typically aluminum. Flexible plastic hosing  is used deliver oxygen to the masks; its use is increasing in  permanent installations to save weight.  The ignition point for any substances is lower in pure oxygen  than it is in air. When high-pressure oxygen is allowed to  rush into a low-pressure area, its velocity could reach the  speed of sound. If it encounters an obstruction (a valve  seat, an elbow, a piece of contaminant, etc.), the oxygen  compresses. With this compression, known as adiabatic  compression (since it builds so quickly no heat is lost to its  surroundings), comes high temperature. Under pressure, this  high temperature exceeds the ignition point of the material  the oxygen encounters and a fire or explosion results. A  stainless-steel line, for example, would not normally burn  and is used for carrying numerous fluids under high pressure.  But under high pressure and temperature in the presence of  100 percent oxygen, even stainless steel can ignite.  Installed oxygen tubing is usually identified with color- coded tape applied to each end of the tubing, and at specified  intervals along its length. The tape coding consists of a green  band overprinted with the words “BREATHING OXYGEN”  and a black rectangular symbol overprinted on a white  background border strip.  [Figure 16-26] Oxygen Plumbing and Valves