Oxygen systems will be one of 3 types: demand, diluter-demand or continuous. System can be a combination of combination of these 3 types and this is often the case. The type of system is determined by the oxygen mask(s) used in the system. For commercial airplanes, crew masks will be able to operate in either the diluter-demand or full demand mode. Passenger masks will generally be continuous flow. Crew masks used in military and space will be demand type of mask. Military masks may also be capable of operating in a demand-diluter mode.
This module discusses demand/diluter-demand masks to describe how a demand type of system operates.
A demand mask only flows oxygen when the user demands flow, i.e., when the user inhales. A demand mask consists of the mask piece that fits over the nose and mouth, an oxygen inlet, an oxygen pressure regulator, an oxygen inlet “demand” valve and an outlet/exhalation valve. A demand mask is shown in Figure 1 and Figure 2.
Figure 1 Demand / Diluter Demand Oxygen Mask without Smoke Goggles
Figure 2 Demand / Diluter Demand Oxygen Mask with Smoke Goggles
For a demand system to work properly, the mask must maintain a positive pressure above ambient pressure to ensure oxygen flows into the lungs. To achieve the positive pressure, the mask must cover both the nose and mouth, and maintain a tight seal against the face. Crew masks meet this criterion. As a side note, studies have chosen that facial hair (i.e., beards) can affect the mask seal against the face and reduce oxygen flow during inhalation – some masks work better than others in this regard. Full demand masks are required above 34,000 feet cabin altitude to meet the minimum mass flow of required oxygen.
The mask has a valve at the oxygen inlet and another valve that opens during exhalation to let the air out. Both of these valves act like a spring loaded check valve, which only open in one direction and require a certain amount of delta pressure before they will open. In practice, this valve is basically a diaphragm that moves along an axis when opening and closing. The diaphragm moves according to the delta pressure across the diaphragm. When inhaling, the pressure in the tracheal (and lungs) drops so that oxygen in the mask flows into the lungs. Oxygen from the pressure regulator is greater than the pressure in the mask, so that oxygen will flow into the mask during breathing.
Key to performance of a demand mask is the pressure regulator on the oxygen inlet. The pressure regulator drops the oxygen pressure to a level that (1) is not high enough to open the demand valve when the user is not inhaling and (2) supplies sufficient oxygen through the valve to meet the minimum oxygen flow requirements when inhaling. TSO-C78A provides requirements for flow rate as a function of delta pressure across the valve for both the inhalation valve and the outlet exhalation valve. For example, when the inlet valve has a pressure drop of 0.6 in H20, the flow rate should be a minimum of 20 liters per minute (LPM). TSO-C78A also states that the exhalation valve must open when the pressure inside the mask (when sealed against the face) is 20 mm Hg and the pressure in the oxygen supply tube is 15-19.9 mm Hg.
Another feature that may be incorporated into demand masks is an emergency oxygen supply. This supply would provide oxygen in event of an ejector or crew egress (such as parachuting from the airplane) or should a failure occur and the normal oxygen supply be lost. The emergency system consists of a bottle, pressure gauge and attaching hose. These parts are connected to the mask or the pilot’s suit in some way. Pulling a cord or lever activates the emergency system.
Demand masks are normally good up to 40,000 feet. The limitation comes from the human body, where the lungs can only withstand a small differential pressure. Use above 40,000 feet can be tolerated for a very short interval, 2-3 minutes at most. For extended time at altitudes above approximately 50,000 feet a pressure suit is required.
A diluter-demand mask only flows oxygen when the user demands flow (i.e., when the user inhales) but allows ambient air to be mixed in with the oxygen flow. A diluter-demand mask is normally the same mask as a demand mask with this additional operating mode (see Figures 1 and 2). This reduces the amount of oxygen that must be supplied by the oxygen supply allowing a given oxygen supply to last longer. The diluter feature is helpful because full oxygen is not required for altitudes below approximately 34,000 feet and the oxygen supply will last longer.
A diluter demand mask contains the same components as a demand mask but contains a diluter valve assembly. The diluter valve assembly consists of an additional diaphragm valve and an aneroid controlled valve. The aneroid is essentially a sealed bellows that expands and contracts with altitude. The diluter valve assembly connects the mask interior with ambient air. The diaphragm valve is normally closed and opens when a suction pressure (inhalation) is applied to the valve, and subsequently closes during exhalation or when there is neither inhalation or exhalation. A pressure regulator controls the pressure upstream of the diaphragm valve to ensure proper operation over the range of ambient pressures. For the aneroid controlled valve, the aneroid is connected to a proportional bellows that controls valve opening as a function of ambient pressure (or altitude). At lower altitudes the valve will be open further than at higher altitudes allowing more ambient air to flow through the diaphragm into the mask at each inhalation. When the altitude reaches approximately 34,000 feet, the aneroid (bellows) will have closed the diluter valve and full oxygen will be flowing into the mask (this is equivalent to full demand operation). The diluter valve can be overridden or manually shut off at any time by the mask user. This would be the 100% oxygen (demand) mode for the mask. In the normal mode, the diluter valve will function as described above. 100% oxygen mode would be used when an user feels a need for greater amounts of oxygen or there is smoke or toxic fumes present.
Most crew oxygen masks will be a diluter-demand mask. Depending on the airplane’s operational requirement, there will be times under normal operation when a pilot will require oxygen (for example, Part 135 requires a pilot to be using oxygen above 35,000 feet or when one pilot leaves the flight compartment). When oxygen is required for normal operation, it is advantageous to minimize oxygen flow to extend the oxygen supply (bottle) duration. This allows a smaller bottle to be installed to meet worse case flight requirements. The altitude limitations for a diluter-demand mask will be the same as a demand mask, approximately 40,000 feet.