Oxygen above 20,000 feet

If you're thinking about going from an Arrow to a T-310, I hope you've also
thought about burning 30 gallons an hour and the need for recurrent
multiengine training as well as the management of turbocharged engines.

A T-310 may cost $300 an hour (at least compared to my Baron), and there
have been some expensive ADs on the twin Cessnas regarding the exhaust
systems (as I recall).

You should also strongly consider a ride in an altitude chamber to be able
to recognize the signs and symptoms of hypoxia, which can be insidious.

The transition from an Arrow to a T-310 is a pretty big jump in cost and
complexity.

O. Sami Saydjari wrote:
Yes, I think so. So if I use a mask, then is there some upper limit?
What am I misunderstanding here?

As the ambient air pressure decreases, you need to increase the percentage of
oxygen in the air you breath to get the same amount of oxygen into your blood.
The nasal canulae will do this up to a point.

Once you get up to about 20,000', the ambient air pressure is so low that the
canula just can't do the job. You need to go with a full mask. Even there, the
less expensive low pressure masks are only good for something like another
8,000'. Then you need a high pressure mask (such as the military used in WWII).
Those will work up to about 35,000'. Above that, you need either a pressure suit
or a pressurized cabin.

George Patterson
"Naked" means you ain't got no clothes on; "nekkid" means you ain't got
no clothes on - and are up to somethin'.

O. Sami Saydjari wrote:
OK, very helpful answers. But, how does one get a "pressurized mask"?

Here's a low pressure model.
http://www.mhoxygen.com/index.phtml?...product_id=373

Here's this retailer's entire offering.
http://www.mhoxygen.com/index.phtml?...prd_group_id=9

George Patterson
"Naked" means you ain't got no clothes on; "nekkid" means you ain't got
no clothes on - and are up to somethin'.

On 26-May-2005, john smith wrote:

The partial pressure of O2 at FL180 is 50% that of sea level.
As you go higher, the rate of partial pressure change decreases more
rapidly, so that by FL270 it is into single digits.
A pressurized mask is necessary to force the air into your lungs and
into your blood.

Actually, you nailed the main issue -- oxygen partial pressure -- but missed
the precise reason for the pressure mask.

As altitude increases, partial pressure of O2 decreases, as you correctly
state. The O2 PP at sea level is way more than we need, and healthy
individuals can tolerate the air at up to about 14000 ft without much loss
of mental capacity (physical capacity, including visual acuity, is another
matter.) Above that (actually, above 12500 ft), in an unpressurized
airplane, we increase the PP of the O2 we breath by adding supplemental
oxygen. The cannulas and/or masks typically used in light aircraft mix pure
O2 from the bottle with atmospheric air at a ratio that can be somewhat
adjusted (by the flow adjustment) to result in roughly the desired O2 PP.
But these systems are not designed to deliver anything close to pure O2 to
the user. The maximum O2 concentration they CAN deliver will provide
sufficient O2 PP at about 18000 ft for cannulas and about 26000 ft (if I
recall correctly) for masks. Above that, you need a system that can deliver
higher O2 concentrations, up to and including pure O2, which is where the
pressure mask comes in. What it does is provide O2 at slight positive
pressure relative to the outside air to prevent uncontrolled mixing in the
mask. Such masks also cover both nose and mouth to prevent leakage/mixing
that way. It doesn't really "force" the O2 into the user's lungs, nor does
it need to.

Above about 35000 ft even pure O2 will not have sufficient PP for adequate
breathing, so unpressurized aircraft cannot safely operate at anything close
to that level (unless the occupants are equipped with pressure suits).

This also explains why cabin depressurization above 30000 feet is an
emergency requiring (a) the immediate donning by the pilot(s) of a pressure
mask and (b) emergency descent to an altitude where the conventional
emergency masks used by passengers will be sufficient.

--
-Elliott Drucker

You don't normally use a pressure O2 system in anything except jets (for
emergencies) and some high altitude gliders and you don't want to anyway.
Pressure O2 systems are very uncomfortable.

Mike
MU-2

"O. Sami Saydjari" wrote in message
...
OK, very helpful answers. But, how does one get a "pressurized mask"? Is
this a matter of buying a pressurized mask and hooking it to a built-in O2
system (such as might be found on a Cessna Turbo 310), or is there
something special about the 02 system itself that is needed. It sounds
like one needs a special pressure regulator like one might find in Scuba
equipment. I ask this because I want to know what to ask for when I am
looking at Turbo Cessna 310s in terms of an adequate O2 system to make it
up to its 28,000 ft service ceiling.

-Sami


john smith wrote:

Very good answer Ben!
The partial pressure of O2 at FL180 is 50% that of sea level.
As you go higher, the rate of partial pressure change decreases more
rapidly, so that by FL270 it is into single digits.
A pressurized mask is necessary to force the air into your lungs and into
your blood.

Ben Hallert wrote:

I think the other posters have it right. The specific concept behind
it, if I remember my science fiction books correctly, is 'partial
pressure'. With the open cycle (the one that hooks to your nose),
there's an open path direct to the low pressure area through your
mouth. The oxygen enters your lungs through osmosis, and if the
pressure on the inside of the hemoglobin in your pulmonary capillary is
greater then the pressure of the O2 against it, it just won't enter.
Another part of the problem may be that at high enough altitudes, CO2
may no longer be effective at triggering the breathing impulse.
I think the partial pressure issue is probably more relevant.

"O. Sami Saydjari" wrote in message
...
OK, very helpful answers. But, how does one get a "pressurized mask"?
Is this a matter of buying a pressurized mask and hooking it to a
built-in O2 system (such as might be found on a Cessna Turbo 310), or is
there something special about the 02 system itself that is needed. It
sounds like one needs a special pressure regulator like one might find
in Scuba equipment. I ask this because I want to know what to ask for
when I am looking at Turbo Cessna 310s in terms of an adequate O2 system
to make it up to its 28,000 ft service ceiling.

-Sami

Some information!

http://www.mhoxygen.com/index.phtml?...prd_group_id=4

http://www.aerox.com/Pages/masks.html

Ben Hallert wrote:
I think the other posters have it right. The specific concept behind
it, if I remember my science fiction books correctly, is 'partial
pressure'. With the open cycle (the one that hooks to your nose),
there's an open path direct to the low pressure area through your
mouth. The oxygen enters your lungs through osmosis, and if the
pressure on the inside of the hemoglobin in your pulmonary capillary is
greater then the pressure of the O2 against it, it just won't enter.

Partial pressures of the gas in question DO define how oxygentation and
ventilation work, BUT the term you are looking for is "pressure
gradient", the difference between the partial pressures in question.

Another part of the problem may be that at high enough altitudes, CO2
may no longer be effective at triggering the breathing impulse.

Not hardly. In the body, arterial blood leaving the lungs has a partial
pressure of CO in the 35-45 mmHG range. In a "mixed venous sample" which
is in the blood returning to the lungs, this value is even higher. The
majority of healthy individuals use CO2 as the determining factor in
their breathing depth and rate, and this does not vary with altitude.

I think the partial pressure issue is probably more relevant.

If someone here is a doctor or actually KNOWS the answer, feel free to
tell me where I pooched it up. My education comes from the likes of
Del Rey Publishing and Baen Books, not John Hopkins.

Ben Hallert
PP-ASEL


I'm not a doc, just an ICU and ER nurse, Paramedic, and former
firefighter. I didn't go to John's Hopkins either.

Other posts elsewhere in the thread raised other questions I wanted to
address:

With a cannula, the limitation is due to the volume of oxygen that is
useable. In a cannula, once your "nose" is filled with free flowing
oxygen, the excess spills over into the mouth, and also out into the
environment. Increasing the flow rate past the point where the "nose"
fills between breaths only increases the waste.

A pulsed regulator (someone called it a "pusher") is helpful in this
regard because the regulator gives HIGH flow, but only during a breath,
so its OFF when not inhaling. Cuts waste WAY down. May give you a little
extra altitude as well. These devices were originally developed for home
health care type patients to extend their supply of oxygen on their
transport bottles, allowing them to get out of the house longer.

A mask gets its effect from having a larger area being filled with
oxygen between breaths (the entire mouth, and then the area under the
mask). For aviation use, a reservoir mask allows you to get an even
larger charge of oxygen per breath.

Those of you who are talking about "pressure breathing" need to
understand that the pressures involved are VERY miniscule. The pressures
that these "pressure breathing" regulators use is measured in
CENTIMETERS of water, which corresponds to a SMALL fraction of a single
PSI. Anything more than that would cause the lung to pop (barotrauma).
The pressures involved contribute LITTLE to the pressure differential.

What is different about the pressure breathing setup is that NORMALLY,
you WORK to breathe in, and relax to exhale. With this setup, your
inhalation is assisted (slightly, its called "pressure support" in the
medical community) and you have to WORK to exhale.

The tanks in question carry in the neighborhood of 1800-2000 psi. They
are routinely filled to 10% over (2000-2200 psi), and when they are
tested I believe the value is 150%. Adding 15 psi by taking the tank
into outer space (yea.. REALLY at altitude) is a miniscule thing in this
pressure equation. The altitude restrictions are a function of the
delivery device, not the storage tank.

Dave

wrote:
This also explains why cabin depressurization above 30000 feet is an
emergency requiring (a) the immediate donning by the pilot(s) of a pressure
mask and (b) emergency descent to an altitude where the conventional
emergency masks used by passengers will be sufficient.

I believe that emergency descent capability is one of the factors that
limits allowable operating ceilings. Some aircraft could get higher than
their certified ceilings, but are not allowed to do so because getting down
to breathable air before the pax suffocated would involve tearing the wings
off (or some other overspeed disaster).

("Roy Smith" wrote)
[snip]
I believe that emergency descent capability is one of the factors that
limits allowable operating ceilings. Some aircraft could get higher than
their certified ceilings, but are not allowed to do so because getting
down
to breathable air before the pax suffocated would involve tearing the
wings
off (or some other overspeed disaster).


What is the time limit - 3 minutes? Tell me it's not 4 minutes.

What are planes/jets doing (ft/min) in emergency descents - and how low must
they go to get to "breathable air?" What's the reg for breathable air out of
an emergency descent - 18,000 ft?


Montblack

Most jets can do 10,000fpm+. It is going to take a couple of minutes in any
case, that is why it is required for either one pilot to be wearing a mask
or to have quick donning masks immediatly available.

Mike
MU-2

"Montblack" wrote in message
...
("Roy Smith" wrote)
[snip]
I believe that emergency descent capability is one of the factors that
limits allowable operating ceilings. Some aircraft could get higher than
their certified ceilings, but are not allowed to do so because getting
down
to breathable air before the pax suffocated would involve tearing the
wings
off (or some other overspeed disaster).


What is the time limit - 3 minutes? Tell me it's not 4 minutes.

What are planes/jets doing (ft/min) in emergency descents - and how low
must they go to get to "breathable air?" What's the reg for breathable air
out of an emergency descent - 18,000 ft?


Montblack