Oxygen indication encompasses both oxygen quantity indication and oxygen flow indication.
Quantity Indication
Airplanes are not allowed to dispatch with an insufficient quantity of oxygen onboard. Therefore, oxygen quantity indication to the flight crew is required. Indication can be provided through pressure readings or volume readings (volume is usually displayed in liters). Pressure or volume will be displayed on specific flight compartment gauges or through an EICAS display. The method of indication should be obvious and easy to read. Figure 1 shows a MIL-R-83178 standard oxygen pressure regulator and control panel that contains an analog oxygen pressure gauge. This pressure reading is used to determine oxygen quantity and duration (duration is how long in the oxygen will last as a function of number of users).
Flight manual oxygen duration charts will be based on the indication. If oxygen quantity indication is done via a pressure transducer, then oxygen duration charts will be based on the pressure reading. As shown in Oxygen System – Calculations, oxygen quantity is a function of both pressure and temperature. Since pressure drops with temperature, oxygen quantity indications using pressure can lead to conservative indications, since a low temperature must be assumed. Consequently, some systems measure both bottle pressure and temperature. These reading are fed into on onboard computer that computes and displays oxygen volume (typically in liters) for use in duration charts. This method provides a more accurate reading then a pressure reading indication system. Usually, oxygen low indication and overpressure warnings are provided by the indication system (based on either pressure or volume readings).
For systems that have a chemical oxygen generator, oxygen volume indication is not provided. Volume (or duration) capability is set by the capacity of the selected oxygen generator. Chemical oxygen systems contain a preset amount of chemical (sodium chromate). The mass of the chemical determines the length of time oxygen is provided.
If a liquid oxygen system is used, then the oxygen quantity indication is accomplished through a pressure gauge and a capacitance-type sensor for volume. The pressure gauge ensures sufficient pressure exists in the system to provide oxygen to the users. The capacitor is an open capacitor, mounted so that the liquid oxygen is the dielectric material separating capacitor terminals immersed in the liquid oxygen. The capacitor provides a current output proportional to the volume of liquid. The current from the capacitor is applied to a indicator’s bridge circuit, subsequently amplified and used to drive a servomotor which positions a mechanically driven pointer or a electronic display. Indication will be a volume reading, typically in liters.
Flow Indicators
Flow indicators are required for both flight crew and passenger masks. Various methods have been used for flow indication, including blinker, float, in-line poppet, rotating indicator or inflation of an inflatable “bag”. Any method is acceptable as long as the indicator is reliable and easy to read in a given installation. SAE AS 916 specifies flow indication must occur at a minimum of 3 LPM-NTPD.
An oxygen flow indicator is shown on the MIL-R-83178 pressure regulator control panel shown in the Figure 1. This indicator will function similar to an inline flow indicator. MIL-R-83178 specifies indication must indicate flow at 4 LPM at sea level and 8 LPM at 35,000 feet for 100% oxygen mode. If diluter (normal) mode, oxygen indication should occur at 18 LPM for all altitudes.
Figure 1 MIL-R-83178 Diluter Demand Oxygen Crew Mask Pressure Regulator Control Panel
For passenger masks, indication is done by one of two methods. One method is a flow indictor that is installed in the inlet tube to the bag. This flow indicator is normally a green slider that slides out when there is a delta pressure (flow) across the indicator (see Figures 2 and 3). When the green slider can be seen, flow is occurring. If the green slider cannot be seen, then the indicator has slid back into a sleeve and there is no flow. A low rate spring pushes the slider back into the sleeve when there is no pressure drop or flow through the tube.
Figure 2 Inline Flow Indicator
Figure 3 Flow Indicator (Shown in No Flow Position)
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The second method is via a cavity in the oxygen bag (see Figure 4). When oxygen flows into the bag, the oxygen flows into the cavity, which will expand the cavity, before flowing into the remainder of the bag (see Figure 4). When the cavity in the bag expands, the user can easily tell there is oxygen flow.
Figure 4 Passenger Bag Flow Indicator