This module discussed normal pressure regulators plus altitude compensating pressure regulators. Figure 1 shows locations of both pressure regulators in a typical system. The main system pressure regulator is located on the oxygen bottle. The altitude compensating pressure regulator is located just downstream of the passenger mask shut off valve.
Figure 1 Standard Oxygen System
System Pressure Regulator
In most oxygen systems, pressure regulators are required to reduce the oxygen supply pressure to a more manageable pressure at the crew or passenger mask. In a typical commercial aircraft oxygen system, the oxygen supply bottle pressure will be in the 1800-2000 psi range. However, crew and passenger masks will be rated in the 70 psig inlet pressure range. Hence the bottle pressure must be lowered to be approximately 70 psig so that expected mask flow rates will be achieved. Pressure regulators are not required for chemical oxygen generators or liquid oxygen generators if the oxygen pressure from the generator is in the required range.
Pressure regulators may be located (1) at the bottle or other pressure source, (2) somewhere in the system between the bottle or pressure source and the mask, or (3) at the mask. Incorporating a pressure regulator at the bottle or pressure source is the best method since it lowers the pressure in the downstream lines resulting in oxygen lines that are designed for a lower pressure (so lighter) and reduces the impact of a burst line. If a pressure regulator is located downstream of the bottle then some of the oxygen lines will see 1800 psi and will need to be designed accordingly. The impact of a burst line at this pressure will need careful examination during system design. Locating a pressure regulator at the mask would have the same issues as locating a pressure regulator downstream of the oxygen pressure source. Nevertheless, some crew masks will contain pressure regulators in either the mask or box assembly to assure optimum flow under all conditions. Multiple pressure regulators may be used in a single system. For example, a 70 psi regulator could be located at the bottle and an altitude compensating pressure regulator could be located downstream of the passenger system shut off valve. Or a pressure regulator could be located at the bottle and each crew mask could contain a regulator.
Note: Passenger systems are isolated from the crew system via a shut off valve so the crew and passenger systems will be independent systems.
Pressure regulator design is governed by industry or military specifications. A commercial specification is SAE AS 1248, Minimum Standard for Gaseous Oxygen Pressure Reducers. A military specification is MIL-R-83178, Regulator, Oxygen, Diluter-Demand, Automatic-Pressure-Breathing, CRU-73A, General Specification for. A schematic view of a pressure regulator and control panel from MIL-R-83178 is shown in Figure 2. This panel shows the various controls that are required on a panel mounted pressure regulator. The Figure 2 regulator panel would be used with a crew mask.
Figure 1 MIL-R-83178 Diluter Demand Oxygen Crew Mask Pressure Regulator Control Panel
Altitude Compensating Pressure Regulators
An altitude compensating pressure regulator will reduce the regulator outlet pressure at lower altitudes and increase the regulator outlet pressure as altitude increases. This has the affect of reducing oxygen flow at lower altitudes (but still above minimum requirements) thus allowing a given oxygen supply to last longer. For example, an altitude compensating pressure regulator may follow a schedule similar to the one below:
Outlet Pressure (psi)
25 – 30
40 - 45
*Regulated pressure value and flow rate is an example only. Actual regulated pressure and corresponding flow rate for a given regulator will depend on the size of a regulator and the settings for a given regulator. The flow requirements depend on the number of continuous flow masks that the regulator supplies oxygen.
To be able to regulate to the pressures listed in the table, the inlet pressure to altitude compensating pressure regulator needs to be greater than 60 psig, preferably around 70 psig.
The altitude compensating pressure regulation is accomplished through an aneroid controlled diaphragm that controls a pilot valve. The pilot valve is held in the high pressure / high flow position by high ambient pressure acting on the diaphragm that overrides a spring to maintain the high pressure / high flow position. As the ambient pressure drops, the spring force will push back against the diaphragm pushing the pilot valve to the open more (increases flow area through the valve), which allows more flow and decreases the pressure drop through the regulator.