Passenger masks are shaped like a cup and designed to fit loosely over a person’s nose and mouth. Passenger masks are generally lightweight and inexpensive. A picture of a passenger mask is shown in Figure 3. Passenger mask assemblies are normally continuous flow devices. This leads to more flow than is theoretically required. Since oxygen is continuously flowing, the bag collects oxygen flow between each inhalation. Passenger masks fit loosely and thus don’t provide a tight seal around the mask periphery. Consequently there is airflow around the mask periphery to/from ambient air. Oxygen will not normally flow around the periphery since the oxygen collects in the bag and is then pulled from the bag at each inhalation.
Figure 1 Passenger Oxygen Mask
A typical passenger mask has an oxygen inlet, an ambient air inlet valve and an outlet (exhalation) valve. Referring to Figure 1, oxygen flows continuously through the hose into the bag. An orifice at the inlet of the hose controls maximum flow rate to the mask. This orifice is calibrated to ensure the minimum mass flow of supplemental oxygen is met under all conditions (cabin altitudes). The oxygen flows continuously into the bag. When a breath is taken, oxygen is pulled from the bag into the mask (and subsequently the lungs) along with ambient air through the ambient air inlet valve. During exhalation, air from the lungs is expelled through the outlet valve and also around the edges of the cup (the amount of leakage depends on how snugly the cup fits on a person’s face). The inhalation and exhalation check valves are located on the front face of the passenger mask as shown in Figure 2. Passenger masks of the type shown in Figure 1 are designed to the requirements of TSO C64A and specification SAE AS 8025.
Figure 2 Continuous Flow Oxygen Mask Valves
Passenger oxygen mask flow is usually greater than required at lower cabin altitudes. A continuous flow mask will be rated at a certain flow rate for a given inlet pressure at normal temperature and pressure, dry (NTPD) conditions. For example, a passenger mask may be rated for 4.5 LPM with a 70 psig inlet pressure to the orifice at NTPD conditions. This information is sufficient to determine the flow at different cabin altitudes (see Oxygen System – Calculations). The rated flow rate is sized to meet the minimum flow requirements at the maximum operating altitude for the mask. Oxygen flow rate is higher at higher altitudes (lower ambient pressures). As altitude drops (ambient pressure increases) and the required oxygen flow rate is less however a continuous flow mask will still flow approximately the same amount of oxygen. For example, the oxygen flow requirement at 15,000 feet is approximately 0.71 LPM-NTPD, which is much less than a 4.5 LPM-NTPD rated flow rate. Hence, continuous flow masks waste oxygen at lower altitudes. In some systems, an altitude compensating pressure regulator is used to regulate the inlet pressure to the passenger mask (orifice inlet). See Oxygen System – Pressure Regulators for more details.
For more information on the operation of passenger masks, see Oxygen System – Continuous Flow.