Wanderlust World Wanderlust World Constant flow air systems are the most common and least expensive type of on-board oxygen system in airplanes. These systems consist of three parts: the cylinder, the regulator and the manifold. The cylinder stores the oxygen, usually at around 2,200 pound per square inch (psi). The regulator changes the pressure of the gas as it runs from the cylinder, usually to between 20 and 75 psi. The regulator's job is to ensure that the pressure coming from the cylinder stays the same regardless of the pressure inside the cylinder.
Both diaphragm and piston regulators can operate oxygen systems. Diaphragm regulators are more common because they can maintain the same pressure regardless of the cylinder, while piston regulators can be affected by the oxygen level in the cylinder. The manifold is what finally delivers the oxygen to the cabin. The advantages of constant flow systems are low maintenance and low weight. However, they can sometimes result in wasted oxygen.
Altitude adjustable on-board oxygen systems are very similar to constant flow systems. However, while constant flow systems provide the same pressure throughout the flight, altitude adjustable systems feature a control that allows the flight crew or pilot to change the pressure. This can be useful as a plane changes altitude. These types of systems are relatively rare, especially in permanent installations. This type of system is expensive, and can only be used to change the air flow in the entire system, and not for individual oxygen masks.
Altitude compensating systems basically function in the same way as altitude adjustable systems except for a single feature: altitude compensating systems automatically change the cabin's oxygen pressure as an aircraft changes altitude. Once again, however, this system only changes the air flow for the entire system, and not for individual breathing apparatuses.
Demand systems are extremely rare. Demand on-board oxygen systems only run if the person wearing a breathing apparatus inhales; a small pulse of oxygen is released when the person inhales. As the plane gains elevation, the length of the pulse increases. This type of system can increase the life of an oxygen cylinder considerably.