QNH In Cruise; And Human QNH Tolerance

Wed Feb 07, 2001 9:01 pm

What is the QNH in an aircraft during cruise? I heard it's around 700 hPa but that sounds a little low.

And, does anybody know the upper and lower limits of air pressure that an ordinary person can sustain?
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RE: QNH In Cruise; And Human QNH Tolerance

Thu Feb 08, 2001 1:00 am

Most pressurized airliners cruise with a cabin altitude of about 8,000 feet.

Given time to adjust and an adequate oxygen supply, human beings can survive in a vast range of pressures. A scuba diver at 100 feet (~30m) is under a pressure of about 10 atmospheres and climbers reach 29,000 feet(~9,500m) on Everest where the pressure is probably 1/10 to 1/5 atmosphere. Those are fairly common situations. I suspect the records far exceed them.

aaron atp
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Joined: Sun Mar 19, 2000 1:17 pm

Breathing Air...

Thu Feb 08, 2001 4:17 am

Upper Limits:

It is commonly held that a .12 partial pressure of oxygen (in atmospheres) is the threshold for serious hypoxia. (.10 atm is below the threshold for life support)

The stated .12 atm PP of O2 breathing air occurs at 15000msl, which -of course- coincides with the applicable FAR. (.10 atm PP of O2 would occur at 20,000 ft)

Breathing Pure O2 at around 48000msl would be the upper limit for breathing pure O2 at the ambient atmospheric pressure. The partial pressure of O2 would be 100% of the atmospheric pressure, which is .12 atm at that altitude

Lower Limits

A 2.82 atm partial pressure of oxygen (pure O2 at a 60 fsw equivalent in a hyperbaric chamber) is the medical limit for administering O2 to DCS sufferers. This can only be tolerated by the body for around ten minutes. At these concentrations, O2 will begin to oxidize lung and nervous system tissues. When that happens, it is called Pulmonary or CNS Oxygen Toxicity.

The non-decompression limit for -recreational- divers breathing air is 132 feet of salt water and eight minutes at that depth. That's 8 minutes from beginning the descent until beginning the ascent. That would mean only 1 minute or so at a depth of 132fsw (which is 5 atmospheres btw), depending on how fast you sink. That limit is based on nondecompression dives, and the limiting factor is nitrogen absorption. If we switch to saturation diving, the navy exceptional exposure limit is 1.8 atm PP of O2. That would occur at 8.5 atm breathing air, or 125psi which equates to ~250 feet of salt water.

If there are any rec/nitrox divers who don't understand this, email me.


Scuba Diving

Thu Feb 08, 2001 6:16 am

At 30m, the scuba diver is not at 10 atmosphere! Pressure change 1atm for every 10 meters, not 3 meters.

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RE: QNH In Cruise; And Human QNH Tolerance

Thu Feb 08, 2001 1:22 pm

Excellent technical stuff aaron. I've even printed it off for reference. Can you give a heads-up as to where it can be found in that amount of detail?

Best Regards,

aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm


Thu Feb 08, 2001 5:00 pm

I really have no clue where you could find that info, it all comes from experience. A short paper I wrote regarding diving and flying covers most of it, which is what I referenced for the oxygen toxicity limits. I'll try to post it to my web page tomorrow.

That paper contains several tables which goes more in depth with most of the info, except for the high altitude remarks. I'll revise it with the info from my previous post and make it available sometime tomorrow.


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