Aircraft Oxygen Equipment — Page 269, Lesson 341

Let’s start with the continuous flow oxygen system, because that’s the foundation everything else builds on. When the shut-off valve and the line valve are turned on, high pressure oxygen flows from the charged cylinder to the Pressure Reducing Valve — the PRV. That valve does exactly what its name says: it reduces the pressure. At the PRV, the pressure is dropped to 80 to 100 psi for supply to the mask connection points. Then, at each mask connection point, the pressure is further reduced by a calibrated orifice — a precisely sized hole — so that oxygen is delivered at the correct pressure for breathing at a continuous rate when required. Now, the mask connection points come in two types. The normal plug-in type, where you physically plug your mask in. And the drop out type, where, in the case of a pressurization failure, the masks are presented automatically and oxygen flow will commence when the passenger puts the mask on. So the drop out type is the automatic presentation for passengers. For the crew and passengers, there are two kinds of continuous flow regulators. The hand adjustable type for crew, and the automatic type for passengers. The hand adjustable regulator delivers a continuous stream of oxygen at a rate you can control. It usually has a pressure gauge, a flow indicator, and a manual control knob used to regulate the flow according to cabin altitude. The gauge indicates the pressure in the cylinder in psi. The flow indicator is calibrated in terms of cabin altitude. So you set the knob based on what the cabin altitude reads. One critical point about flow indicators: they show that oxygen is flowing through the regulator. They do not show how much is flowing, and they do not show if the user is being supplied with sufficient oxygen. So a flow indicator is a "yes, it's flowing" light, not a quantity or adequacy meter. Now let's move to the Diluter Demand System. This is the system provided in most aircraft for flight crew use, and it is separate and additional to the passenger system. Oxygen is diluted with air and supplied as demanded by the user's respiration cycle and the oxygen regulator. There is a mask connection point for each crew member and the supernumerary crew position — that's an extra crew position beyond the minimum required, like a jump seat. A typical regulator operates like this. With the oxygen supply ON and NORMAL oxygen selected, diluted oxygen is supplied to the crew member's mask as he or she inhales. As cabin altitude increases and cabin air pressure decreases, the percentage of oxygen increases until, at 32,000 feet cabin altitude, 100% oxygen is supplied. So the higher you go, the richer the mix, topping out at full oxygen at 32,000 feet. If the crew member selects 100% O2 on the regulator control panel, 100% oxygen is supplied regardless of altitude. Selecting EMERGENCY on the regulator provides protection against the inhalation of smoke and harmful gases by supplying 100% O2 at a positive pressure — positive pressure means the gas is pushed into your lungs, which keeps smoke out. And when TEST is selected, oxygen at a high positive pressure is supplied to check masks for fit and other equipment for leakage. So the key contrast: the continuous flow system just streams oxygen at a set rate, while the diluter demand system meters oxygen based on your breathing and altitude, with those four selector positions — NORMAL, 100% O2, EMERGENCY, and TEST.