# Why does this oxygen duration chart have increased times above 30,000 FT MSL?

This oxygen duration chart from an airplane shows the amount of time a pilot can be on oxygen at a specified altitude (in 1000s of feet). For example, the chart shows two pilots able to cruise at a cabin altitude of 35,000 FT MSL for 182 minutes using a diluter demand system or 192 minutes using 100% oxygen.

I have one question.

1. Why does the oxygen duration chart show increasing times above 30,000 FT MSL?

• “Why does using 100% oxygen provide greater time...” It does not, the bold numbers (for 100% oxygen) are all lower than the lighter face numbers - as explained at the bottom of the chart. Commented Sep 5, 2020 at 0:43
• You're right. I've updated the question. Commented Sep 7, 2020 at 14:38
• I wonder if someone on the Physics stack would know. I can't think of a biological reason this would be the case, but maybe it has to do with the physical properties of the gas in the storage containers. Like, maybe at lower altitudes, more of the stored oxygen remain in the system because of the higher atmospheric pressure. Commented Sep 8, 2020 at 13:13
• The bottle is regulated to output 75psi to all the passengers on the plane. It could be the difference between PSIA and PSIG. I'll ask them. Thanks. Commented Sep 8, 2020 at 15:46
• This might be worth asking on the aviation stack...
– John
Commented Oct 11, 2020 at 3:44

This is related to the diluter demand system, O2 present in the cabin, and tracheal pressure.

The diluter demand system is designed to compensate for the short-comings of the continuous flow system. It gives the user oxygen on-demand (during inhalation) and stops the flow when the demand ceases (during exhalation). This helps conserve oxygen. Additionally, the incoming oxygen is diluted with cabin air and provides the proper percentage of oxygen, depending on the altitude. This system is typically used at altitudes up to 40,000 feet.

When you get at altitudes higher than 35,000 feet you're more or less totally reliant on the oxygen provided by the diluter, that's why the diluter demand is very close to the cruise time. If there's low air pressure in the cabin above 35,000 feet there won't be enough oxygen for normal body function, and you run the risk of becoming hypoxic within half a minute.

Toe of Useful Consciousnees (TUC) per Effective Performance Time (EPT)

Look at the chart below:

Tracheal oxygen partial pressure starts at 149mm. Hg for sea level and can drop to 20 at 40,000 feet. If oxygen is given according to the % supplement O2 required for inspired air, the tracheal pressure stabilizes back to what it is at sea level. Above 30,000 feet, tracheal oxygen partial pressure cannot be maintained at 149mm. Hg. Therefore you need to use the diluter longer to at least maintain it close to 149mm. Hg. Or at level where the mm. Hg. isn't extremely low.

The required mean tracheal oxygen partial pressure can be different at the same altitude depending on the liters of O2 per minute BTPS (Body Temperature and Pressure Saturated) being inspired.

Fore example, according to 14CFR 23.1443:

(ii) 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.

(2) For each flight crew member, the minimum mass flow 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.

So, at above 30,00 feet you will use most of the diluter demand oxygen to preserve tracheal pressure until the plane has stabilized. At lower altitudes the diluter demand is lower because you can reach safer altitudes faster.

• Doesn't this mean you should use more oxygen above 30,000 FT MSL? Commented Sep 12, 2020 at 16:25
• The % required O2 oxygen (inspired) needs to be 100%. That is because if you use the diluter system the air in the cabin is too thin to provide any meaningful O2. For the diluter system the incoming oxygen is diluted with cabin air and provides the proper percentage of oxygen. At 30,000 FT MSL oxygen in the air too thin and you're going to use more of O2 provided by the diluter. Commented Sep 12, 2020 at 20:35
• Why then does the chart show in an increase in oxygen available time above 30,000 FT? Commented Sep 13, 2020 at 13:19
• Oxygen in the air will be too little but the oxygen available from the diluter will increase above 30,000 FT. O2 is too thin at that level to maintain tracheal pressure and % saturated O2 in the blood the availability of O2 will increase. If you're flying at 35,000 for 182 minutes, you're going to need 182 minutes on diluter system and some extra so that the shift to lower altitudes minimizes the effects of altitude acclimatization. Commented Sep 13, 2020 at 18:37
• I wonder if the physics stack had a different answer, or a better one. If they did and you could provide a link I would love to see what they answered with. Commented Sep 13, 2020 at 18:40