Why does density altitude not affect oxygen concentration?

[kicktireslightfires]

Oxygen requirements are based on pressure altitude, right? Why not based on density altitude? When it’s hotter out, the air is thinner and the plane suffers in performance as a result; the engine makes less power because there’s less air, the wings develop less lift. But the oxygen remains the same? Trying to understand. If the airplane’s performance is based on density altitude, which isn’t the human body’s performance also based on density altitude?

 

[SebIp]

ABC of oxygen: Oxygen at high altitude

www.ncbi.nlm.nih.gov

When you look at the altimeter it is showing you the pressure. Which is all that you need to know regarding oxygen intake in your lungs.

The DA is more about additional mechanical performance.

 

[OneCharlieTango]

FWIW, it isn’t about oxygen concentration; it’s about pressure. Specifically, partial pressure of oxygen. It has to do with the oxygen-hemoglobin dissociation curve, blah, blah, mumble…..

Anyway, the partial pressure of O2 is independent of density altitude.

 

[Capt. Geoffrey Thorpe]

The difference between pressure and density altitude is temperature and humidity - but inside your lungs the temperature is near body temperature and the humidity is very high - so changes in the outside temperature and humidity would have only small effects.
Or, you can look at it as Mr. Tango suggests.

 

[flyingron]

Your ability to take up oxygen depends on the partial pressure across the cell membrane not density. Further, your respiratory tract humidifies the air so you don't even have the variable of humidity to worry about.

 

[Domenick]

Another way to state what has already been stated is, the concentration of oxygen is constant through the atmosphere and at all altitudes, about 21%. However, the absolute O2 content of air decreases with decreasing pressure.

This is analogous to 100% humidity and 21% O2. Altitude (pressure) affects O2 content in the same way temperature affects water content. At very cold temps, even 100% humid air contains very little water, and as air pressure decreases 21% O2 air contains decreasing absolute amounts of O2.

 

[chemgeek]

What you need to know is d=PM/RT, where d is density, P is pressure, M is molar mass (29 g/mol for air), T is temperature, and R is the gas constant. Air density, which is inversely proportional to temperature, is what influences lift and engine power (more molecules per unit volume to provide lift or provide air for combustion). Pressure is what determines the available oxygen for biological processes. The partial pressure of oxygen is 21% of the total pressure of air. Note that both pressure and temperature affect air density, whereas the partial pressure of oxygen is only affected by the ambient pressure.

 

[GaryM]

The partial pressure thing works in the other direction, too. O2 becomes toxic under pressure, so technical and commercial divers breathe mixes with less O2 as they go deeper. Air (21% O2) starts to pose a risk of O2 toxicity around 180-200 FSW, though by then the narcotic effect of nitrogen is also a problem. Substituting in helium solves both.

I've known wreck divers working the really deep stuff to use bottom mixes with just 4% O2, though you can't breathe that at the surface; you need higher O2 travel mixes to get you down to those depths. I'm a lightweight, I never dove anything requiring less than 16% O2, which is fine on descent from around 30 to 250 FSW, followed by decompression on high O2 mixes.

 

[OneCharlieTango]

That’s very cool, Gary.

 

[Initial Fix]

The percentage of oxygen in air doesn’t change until above what we ga pilots will ever fly. It’s 21% O2 until very high. Granted the partial pressure certainly goes down.

Your plane is seeing the lower pressure associated with the higher density altitude. That is just putting less total air volume into the cylinder which affects the total fuel air mixture. Thus lower performance.

 

[luvflyin]

Oxygen requirements are based on pressure altitude, right? Why not based on density altitude? When it’s hotter out, the air is thinner and the plane suffers in performance as a result; the engine makes less power because there’s less air, the wings develop less lift. But the oxygen remains the same? Trying to understand. If the airplane’s performance is based on density altitude, which isn’t the human body’s performance also based on density altitude?

Pressure altitude is just a common point of reference. They'd take into account the range of actual pressures there could be when they set the altitudes in the rules. That's my take on it.

 

[Clip4]

The regulation referee to cabin pressure altitude, not indicated or pressure altitude.

 

[cowman]

It’s also worth noting that density altitude is something you can just plug into your aircraft’s performance charts and get a takeoff/landing distance.

The same charts don’t exist for individual humans. Some say they’ve gone well above 12,500 without O2 and been fine. There are others who say they’re feeling fatigue above 8-10,000. The FAA had to draw a line somewhere so they picked a pressure altitude of 12,500.

 

[Pinecone]

Actually the 12500 for O2 was not based on physiological needs. It was based on the altitudes flown before pressurized airliners.

When I was a USAF student pilot, one exercise was to remove your mask at FL250 and explore your hypoxia symptoms. I was the guy that had symptoms, but was function for a long time. So long, they gave up on me and told to go ahead and put my mask on.

Now days, I run a Mountain High O2D2 and set it to D5, so over 5000 feet pressure altitude, I start getting a bit of O2