Minimum mass flow requirements for oxygen systems are historically based on military testing and evaluations conducted 40 or 50 years ago. Results from this activity evolved into commercial and military specifications. The requirements have remained unchanged for many years and are specified in terms of the partial pressure of oxygen at a condition of body temperature, pressure, saturated (BTPS). The oxygen partial pressure requirements vary with altitude.
This testing found that it was much easier and more consistent to measure tracheal (throat) pressure rather than the pressure in the lungs. Consequently minimum mass flow requirements are specified in terms of tracheal pressure. Tracheal pressure and lung pressure are nearly identical, but of course tracheal pressure will be slightly higher than lung pressure.
For commercial and general aviation aircraft, the minimum mass flow requirements are specified in FAA 14CFR 23.1443 and 25.1443 regulations. For the discussion here, 25.1443 will be used as a reference and is listed below.
14CFR Part 25, 25.1443 Minimum Mass Flow of Supplemental Oxygen
(a) If continuous flow equipment is installed for use by flight crewmembers, the minimum mass flow of supplemental oxygen required for each crewmember may not be less than the flow required to maintain, during inspiration, a mean tracheal oxygen partial pressure of 149 mm. Hg. when breathing 15 liters per minute, BTPS, and with a maximum tidal volume of 700 cc. with a constant time interval between respirations.
(b) If demand equipment is installed for use by flight crewmembers, the minimum mass flow of supplemental oxygen required for each crewmember may not be less than the flow required to maintain, during inspiration, a mean tracheal oxygen partial pressure of 122 mm. Hg., up to and including a cabin pressure altitude of 35,000 feet, and 95 percent oxygen between cabin pressure altitudes of 35,000 and 40,000 feet, when breathing 20 liters per minute BTPS. In addition, there must be means to allow the crew to use undiluted oxygen at their discretion.
(c) For passengers and cabin attendants, the minimum mass flow of supplemental oxygen required for each person at various cabin pressure altitudes may not be less than the flow required to maintain, during inspiration and while using the oxygen equipment (including masks) provided, the following mean tracheal oxygen partial pressures:
(1) At cabin pressure altitudes above 10,000 feet up to and including 18,500 feet, a mean tracheal oxygen partial pressure of 100 mm. Hg. when breathing 15 liters per minute, BTPS, and with a tidal volume of 700 cc. with a constant time interval between respirations.
(2) At cabin pressure altitudes above 18,500 feet up to and including 40,000 feet, a mean tracheal oxygen partial pressure of 83.8 mm. Hg. when breathing 30 liters per minute, BTPS, and with a tidal volume of 1,100 cc. with a constant time interval between respirations.
(d) If first-aid oxygen equipment is installed, the minimum mass flow of oxygen to each user may not be less than four liters per minute, STPD. However, there may be a means to decrease this flow to not less than two liters per minute, STPD, at any cabin altitude. The quantity of oxygen required is based upon an average flow rate of three liters per minute per person for whom first-aid oxygen is required.
(e) If portable oxygen equipment is installed for use by crewmembers, the minimum mass flow of supplemental oxygen is the same as specified in paragraph (a) or (b) of this section, whichever is applicable.
This regulation provides oxygen flow requirements in terms of mean tracheal oxygen partial pressures under BTPS conditions. These oxygen partial pressures are required to ensure oxygen in the lungs is absorbed into the blood and that there is sufficient oxygen in a given volume of air. Different oxygen partial pressure requirements are provided for crew masks, passenger masks, first aid oxygen equipment and portable oxygen equipment.
The percentage of oxygen contained in air is equal to the partial pressure of oxygen divided by the total air pressure. Thus specifying oxygen partial pressures is equivalent to specifying the percentage of oxygen in a given volume of air. The flow rate of 15 LPM, 20 LPM or 30 LPM is the flow rate of air that would be breathed during normal breathing. For example, in one minute at 15,000 feet a passenger would breathe 15 liters of air into their lungs (and also expel 15 liters). Tidal volume is the amount of air that enters the lungs at each inhalation and exits at each exhalation. The regulation states that each breath will be 700 cc (in actual mask use, the respiration rate and tidal volume will vary with each user, so the regulation provides a minimum value). The implication for the tidal volume is that the oxygen mask must contain the required volume of air with the required oxygen percentage (stated in terms of oxygen partial pressure). During certification of oxygen masks to FAA Technical Standard Orders (TSOs), the mask manufacturers ensure this requirement is met. As a result only masks that have been certified to meet the requirements of the applicable TSO should be used in oxygen systems. In commercial aircraft, crew masks are designed to the requirements of TSO C78 and passenger masks are designed to TSO C64A.