Anybody using oxygen generators?

[peter-h]

I've been flying with oxygen for years, and have now settled on the excellent Mountain High kit shown at the end of that writeup.

However a colleague prompted me to look at stuff like this, which costs about $2k. It uses zeolite to absorb the nitrogen, so you get about 90% oxygen instead of 21% you normally get. Some background is here.

These products go for down to $200 but reportedly most of them stop working slightly over 10,000ft. They are meant to work in airliner cabins i.e. up to 8500ft.

The Sequal one has been tested at FL200, by a pilot known to me, and worked fine.

The unit also has an electronic demand regulator, but that's obviously no good if used by more than one person, so one would use the constant flow mode, either with an oximiser cannula or with the O2D2 electronic regulator.

The power consumption at 24V is about 5A, so one would need a power outlet installed with a 10-15A CB.

The output pressure, according to the wiki article, is probably about 20psi which is in the right range for the O2D2 regulator, but this will probably fall with altitude, whereas the output pressure of a 1st stage regulator on an oxygen cylinder will remain constant and I think the O2D2 assumes that in the way it varies the pulse width according to the altitude.

 

[flyingron]

At 3L max for the EXPENSIVE one, I'm not sure how well it will work with the MH or Precise demand regulators. They expect a pretty high flow rate while they're flowing.

 

[peter-h]

What I have is this:

The FAA requires that all pilots flying their aircraft above 12,500 feet for 30 minutes or longer or at 14,000 feet or above during the entire flight must use supplemental oxygen.

Passengers must have oxygen available (but don't need to use it) above 15,000ft. The amount required (reference: FAR 23.1447) is 1 liter of oxygen per minute for every 10,000 feet.

For example, at 18,000 feet there should be a flow of 1.8 liters per minute of oxygen available via a standard breathing device.

So 3 litres/minute should cover 2 people at 15000ft with constant flow cannulas, which nobody uses. With an oximiser cannula, you are at least 2x better (i.e. four people at 15000ft) and with the O2D2 at least 2x better still. Test flight data (Mountain High have seen that article).

What I don't know is whether the FAA's "1 litre per minute per 10000ft" requirement is enough to achieve say 95% blood oxygenation.

 

[Doggtyred]

What I have is this:

The FAA requires that all pilots flying their aircraft above 12,500 feet for 30 minutes or longer or at 14,000 feet or above during the entire flight must use supplemental oxygen.

Passengers must have oxygen available (but don't need to use it) above 15,000ft. The amount required (reference: FAR 23.1447) is 1 liter of oxygen per minute for every 10,000 feet.

For example, at 18,000 feet there should be a flow of 1.8 liters per minute of oxygen available via a standard breathing device.

So 3 litres/minute should cover 2 people at 15000ft with constant flow cannulas, which nobody uses. With an oximiser cannula, you are at least 2x better (i.e. four people at 15000ft) and with the O2D2 at least 2x better still. Test flight data (Mountain High have seen that article).

What I don't know is whether the FAA's "1 litre per minute per 10000ft" requirement is enough to achieve say 95% blood oxygenation.

I did the math many many years ago and in a healthy person, 1 liter/min per 10,000 ft is overkill and more than adequate (disclaimer - in GA environments - I wouldn't try to set any records for high altitude flight in an unpressurized cabin)

 

[flyingron]

Below 25,000 you are probably right. The problem is once you start flowing more than 2-4 l/m in a canula you've passed the point of diminishing returns anyhow.

 

[bbchien]

What I have is this:

The FAA requires that all pilots flying their aircraft above 12,500 feet for 30 minutes or longer or at 14,000 feet or above during the entire flight must use supplemental oxygen.

Passengers must have oxygen available (but don't need to use it) above 15,000ft. The amount required (reference: FAR 23.1447) is 1 liter of oxygen per minute for every 10,000 feet.

For example, at 18,000 feet there should be a flow of 1.8 liters per minute of oxygen available via a standard breathing device.

So 3 litres/minute should cover 2 people at 15000ft with constant flow cannulas, which nobody uses. With an oximiser cannula, you are at least 2x better (i.e. four people at 15000ft) and with the O2D2 at least 2x better still. Test flight data (Mountain High have seen that article).

What I don't know is whether the FAA's "1 litre per minute per 10000ft" requirement is enough to achieve say 95% blood oxygenation.

Peter, there is so much biologic variabiity that FAA and Mountain High can't really publish a table with a straight face. You need a pulse oximeter. Above 18000 feet, just about nobody can maintain 90% on any cannula- you're going to need a mask. That is a a physics limitation, which does apply to me and to every passenger I've ever had above 18,000...

 

[kyleb]

Just for clairification:

Isn't that an oxygen concentrator (not generator)?

 

[X3 Skier]

Just for clairification:

Isn't that an oxygen concentrator (not generator)?

Yes, that's the "official" name.

Cheers

 

[peter-h]

Above 18000 feet, just about nobody can maintain 90% on any cannula- you're going to need a mask

Both myself and my son (16) maintain 95% at FL200, using the O2D2 and plain cannulas.

That's using normal breathing.

That result was suprising, but is real.

 

[bbchien]

Both myself and my son (16) maintain 95% at FL200, using the O2D2 and plain cannulas.

That's using normal breathing.

That result was suprising, but is real.

If you use a recording oximeter a FL 20 for half an hour, it's surprise you. Then do it each day for 3 weeks after you have an upper respiratory....

IF you reduce PaC02 and replace that portion with O2 you can accomplish 200; but you must unconsciously maintain it. There's literally tons of USAF data that describes what you have obeserved, it just isn't going to be maintained. That much respiratory alkalosis is not good for brain blood flow/cognition, etc....the author is Nestus et al at the civil aeromedical institute, and on the xmas day in a ski lodge I've not got the gumption to look it up....

 

[peter-h]

I don't understand what you are saying.

Are you saying that the blood o2 meter is lying, if the altitude is high enough.

I have got those results at FL200 for an hour or two.

 

[gismo]

If you use a recording oximeter a FL 20 for half an hour, it's surprise you. Then do it each day for 3 weeks after you have an upper respiratory....

IF you reduce PaC02 and replace that portion with O2 you can accomplish 200; but you must unconsciously maintain it. There's literally tons of USAF data that describes what you have obeserved, it just isn't going to be maintained. That much respiratory alkalosis is not good for brain blood flow/cognition, etc....the author is Nestus et al at the civil aeromedical institute, and on the xmas day in a ski lodge I've not got the gumption to look it up....

Bruce, are you certain it's not possible to maintain decent o2 blood saturation using electronically controlled delivery through a cannula? It might just be that each normal inhalation delivers nearly the same amount of o2 that breathing through a simple mask provides with very little ambient air (20% o2) mixing in. With any cannula fed by continuous flow (including a "conserving" type) a significant dilution occurs with each breath but it seems plausible that the O2D2 delivery mechanism might provide a much higher concentration of oxygen.

Now if Peter's experience includes deliberately modified breathing I would agree with you that it's not likely he could maintain that continuously for an extended period.

 

[peter-h]

Specifically, what suprised me was that modified breathing was not used.

With my previous systems (oximiser cannulas, or the Precise Oxygen mechanical demand regs) this would not have been possible, and very conscious breathing was necessary above FL160, very much so at FL180, and having to work very hard at it (solo flights only) at FL200.

It was after one such flight at FL200 (just keeping above fairly high convective wx) that I decided to spend the money on the O2D2 regs. The results were astonishing.

However, at FL200, I was not using just the basic "cannula" mode. IIRC, I was using the next one up which I think is the "mask" mode.

I think it's fairly obvious the reason why the O2D2 works this well is because it gives you a pulse of o2 right at the start of the inhalation cycle. You don't get this with the mechanical demand regs (which worked great to ~FL160, on many flights over the Alps etc) and this is obvious because one can hear the o2 pulse relative to one's breathing, and for some reason you don't get it with an oximiser cannula.

 

[SteveinIndy]

For example, at 18,000 feet there should be a flow of 1.8 liters per minute of oxygen available via a standard breathing device. [/I]

So 3 litres/minute should cover 2 people at 15000ft with constant flow cannulas, which nobody uses.

Except that most nasal cannulas are run at 2-4 L/min if you want to have any significant effect on blood oxygen saturation. Slightly more if you're a mouth breather (not meant as an insult, I am being serious here)....

With an oximiser cannula, you are at least 2x better (i.e. four people at 15000ft) and with the O2D2 at least 2x better still. Test flight data (Mountain High have seen that article).

What I don't know is whether the FAA's "1 litre per minute per 10000ft" requirement is enough to achieve say 95% blood oxygenation.

It's probably not. It certainly does not seem like the information on the website you quoted is probably safe to base practices off of. At a minimum, a 95% saturation should be maintained and you're not going to get that at 10,000+ ft AMSL and 2 liters even with an Oxymizer or dividing it between multiple passengers in some sort of a misguided attempt to save weight or cost. Anything below a 95% saturation for any period of time is going to slowly and quietly eat away at ones' cognitive factors (short term memory and basic computational skills are good examples of things that falter with even low-grade hypoxia). In the scheme of things aviation related, oxygen should be a vital concern that should not be skimped on but as far as costs or weight penalty go it is not a major hurdle.

My personal practice is to wear a cannula at 2-4 L/min if above 6,000 ft (which is about where my SpO2 drops to 94 or 95% as it is with most of us "flat landers" who aren't physiologically adapted to living at altitudes) and a mask at 6-8 L/min above 10,000 ft in the rare cases I go that high in an unpressurized aircraft. This is based on what I know about high altitude physiology and experience with oxygen systems from over ten years as an EMS provider and a respiratory therapist.

 

[peter-h]

Except that most nasal cannulas are run at 2-4 L/min if you want to have any significant effect on blood oxygen saturation

a mask at 6-8 L/min above 10,000 ft in the rare cases I go that high in an unpressurized aircraft

The problem is that those statements are debunked every time a pilot flies on oxygen, with modern equipment... by an order of magnitude.

Something is not adding up here...

I did that peter2000.co.uk flight test myself, with another pilot, and we used very careful procedures, and we both got the same results.

EMS provider and a respiratory therapist.

Maybe most of your subjects were unwell?

 

[kyleb]

My personal practice is to wear a cannula at 2-4 L/min if above 6,000 ft (which is about where my SpO2 drops to 94 or 95% as it is with most of us "flat landers" who aren't physiologically adapted to living at altitudes) and a mask at 6-8 L/min above 10,000 ft in the rare cases I go that high in an unpressurized aircraft.

Are you the same SteveinIndy whose profile on the EAA forums says he has <100 hours and who has commented that most of that time is in ultralights?

 

[peter-h]

This site also gives a flow rate per person at FL180 with an oximiser cannula, at about 0.7L/min per person (see the graphic).

 

[SteveinIndy]

Are you the same SteveinIndy whose profile on the EAA forums says he has <100 hours and who has commented that most of that time is in ultralights?

Yes but you have the hours wrong. I have somewhere around 900 hours as PIC and probably another 400 or so as a "right seater" to friends (I don't strictly log or often mention that time since most people don't count dual pilot operations in GA even though I always prefer to fly dual pilot if for nothing other than the company). Granted a lot of the PIC time (about 700 hours) is in ultralights but that is besides the point. When I fly in standard GA aircraft (which is all I do now), I don't putter around at low altitude for reasons of efficiency, etc. We aren't discussing my total time. We are discussing high altitude and cardiopulmonary physiology.

Maybe most of your subjects were unwell?

Maybe we are taught what is normal across the range of environments in school? I also worked doing research using healthy subjects including high altitude physiology. I also have quite a bit of experience with regards to climbing and the exposure of unacclimated climbers to altitude.

The problem is that those statements are debunked every time a pilot flies on oxygen, with modern equipment... by an order of magnitude.

Something is not adding up here...

The reason for higher flow rate in a mask has two aspects to it: The first is actual oxygen delivery and the second is clearing carbon dioxide out of the mask. Putting a tight fitting mask on someone's face with low continuous flow (or low pressure pulse dose) and you are having them rebreath their exhaled gas. That is a primary reason why I suggest and use higher flow rate. While CO2 narcosis is kind of amusing at times, while flying is not one of them.

The amount of flow need for oxygen delivery probably depends on your end points and expectations. You expectation appears to be that 90% is OK. Anything below 94% in most adults is going to start to affect your cognition. This has been shown in both altitude chambers and experiments at altitude (usually in the mountains).

Even then, SpO2 is a proxy measurement at best. What really matters is not the oxygen tension (PaO2) and the cerebral oxygen saturation. Neither of those are easy to measure

I did that peter2000.co.uk flight test myself, with another pilot, and we used very careful procedures, and we both got the same results.

You do realize that extrapolating from two subjects is risky at best? If you submitted it to a research journal, they would tell you to go back and get more data points before resubmitting it. It's too easy to (intentionally or unintentionally) skew the data with two sets of data points. If one of you was anemic (which often goes unrecognized until blood tests are done at the time of a physical), a smoker or had a condition like polycythemia, it could skew the numbers and render them largely meaningless. That is my concern with your testing.

It is good that you took the time to test it. My point is simply that you need to be very careful choosing your endpoints and extrapolating from small sample sizes.

Given how cheap oxygen is, it simply is not worth the risk in the form of cognitive effects and mood changes that accompanies even mild to moderate hypoxia associated with ascent to altitude as pointed out by folks like Bahrke and Shukitt-Hale (1993) in a study carried out for the US military. Van Liere and Stickney (1963) pointed out that the mood effects can range from euphoria to depression and on through apathy, quarrelsomeness, irritability and anxiety. The apathy may actually explain some of the "How did they miss that?" cases we see in GA crashes (overlooking low fuel levels, etc).

 

[peter-h]

I also have quite a bit of experience with regards to climbing and the exposure of unacclimated climbers to altitude.

That's a very different thing though. I've walked up to 16k and it's damn hard, the last 2-3k.

The reason for higher flow rate in a mask has two aspects to it: The first is actual oxygen delivery and the second is clearing carbon dioxide out of the mask. Putting a tight fitting mask on someone's face with low continuous flow (or low pressure pulse dose) and you are having them rebreath their exhaled gas.

I don't recall anyone saying a mask doesn't need a much higher o2 flow rate.

If one of you was anemic (which often goes unrecognized until blood tests are done at the time of a physical)

I've had very comprehesive blood tests done (for a different reason) and don't have that problem. Neither is a smoker or ever has been.

If you submitted it to a research journal, they would tell you to go back and get more data points before resubmitting it

Thankfully I am not

But, hey, the traditional way to deal with awkward data in the medical profession is to suppress it, isn't it? I hear the UK Govt has spent $800M on a stockpile of a flu vaccine which is no better than a placebo

Given how cheap oxygen is, it simply is not worth the risk in the form of cognitive effects and mood changes that accompanies even mild to moderate hypoxia associated with ascent to altitude

Again, nobody is suggesting anything to the contrary.

This is about researching the topic in hand.

I don't see a clear case for discarding an o2 generator because you can always get 99% saturation from a cylinder and a mask, and to hell with how long it lasts before the stuff runs out and you have to run around like a nutter finding somewhere where they will refill it.

 

[SteveinIndy]

That's a very different thing though. I've walked up to 16k and it's damn hard, the last 2-3k.

Not really in terms of physiology. Both that scenario and a flight are considered rapid ascents because they take place so quickly that your body can't acclimate.

As for finding somewhere to refill it, I don't know about the UK but almost every town in the US has a supply of O2. Granted, it may come from a welding shop but they source their gas from the same places as the hospitals and medical supply stores. In many cases, it literally comes out of the same truck as it moves along the route.

Again, nobody is suggesting anything to the contrary.

This is about researching the topic in hand.

Right. Agreed. I wasn't trying specifically to criticize you personally. I was just trying to make sure people are getting the full breadth of information since there is a lot of information about oxygen and altitude. Unfortunately, a lot of it is very bad or based on outdated theories (see the FARs for this).

I don't see a clear case for discarding an o2 generator because you can always get 99% saturation from a cylinder and a mask, and to hell with how long it lasts before the stuff runs out and you have to run around like a nutter finding somewhere where they will refill it.

I often wonder if there might be a benefit to using an oxygen separator (molecular sieve or otherwise) with a higher throughput and just dumping the O2 in the cabin to raise the oxygen level for the whole thing. The nitrogen could even be used to inert the fuel system if one was so inclined.

But, hey, the traditional way to deal with awkward data in the medical profession is to suppress it, isn't it?

LOL


Not in the slightest. The way we deal with it is to replicate the study and see what happens. Usually awkward data or seemingly bizarre results is the result of a poorly constructed study or some unforeseen variable. If you get a truly unexpected result, you are very wise to collaborate with another researcher and have them at least review your data if not independently verify it through a repeated test before publishing it.

I hear the UK Govt has spent $800M on a stockpile of a flu vaccine which is no better than a placebo

Sounds like a bunch of political spin. There is a lot of bad information out there about vaccines (even beyond the disproved autism and MMR vaccine correlation).

 

[kyleb]

Yes but you have the hours wrong. I have somewhere around 900 hours as PIC and probably another 400 or so as a "right seater" to friends (I don't strictly log or often mention that time since most people don't count dual pilot operations in GA even though I always prefer to fly dual pilot if for nothing other than the company). Granted a lot of the PIC time (about 700 hours) is in ultralights but that is besides the point. When I fly in standard GA aircraft (which is all I do now), I don't putter around at low altitude for reasons of efficiency, etc.

Here is a quote from your original post in the thread:

"My personal practice is to wear a cannula at 2-4 L/min if above 6,000 ft (which is about where my SpO2 drops to 94 or 95% as it is with most of us "flat landers" who aren't physiologically adapted to living at altitudes) and a mask at 6-8 L/min above 10,000 ft in the rare cases I go that high in an unpressurized aircraft."

​

Below is a portion of your profile from the EAA Forum. According to the date stamp, you joined the forum in July, 2011, so the information can't be more than 18 months old.

Certificates / Ratings:

• Student pilot

Hours Flown:

• <100

Based on what I quoted above, you now have about 200 hours PIC in an airplane, of which ~50 were almost certainly in a relatively low performance trainer. You stated that you may have 400 hours in the right seat. Your final claim was that it is rare for you to go above 10k in an unpressurized aircraft. Add all of that up and it is apparent that you're a low time pilot with very little experience on O2.

Yet you post in an authoritative manner about O2 use as if you flew 50 missions over the hump.

C'mon man...

 

[SteveinIndy]

That's odd. I don't recall ever filling out that information....then again my accounts on there and on a couple of other forums did get "hacked" a year or so ago (by an ex-girlfriend on whose computer I used to access those forums) so that might explain where the bad info came from. I had to redo the information on my profiles and may have just overlooked the one over there. Thanks for bringing it to my attention.

That said, even if I am going along as a right front seat "passenger" (at least in the eyes of the FAA since I'm not PIC and it's not a dual pilot mandated aircraft), most of my flying has either been below 10,000 feet or in a pressurized aircraft (usually just handling the radios etc as the right seater). Simple enough?

Your final claim was that it is rare for you to go above 10k in an unpressurized aircraft. Add all of that up and it is apparent that you're a low time pilot with very little experience on O2.

Regardless if I am a low time pilot or one with 20,000 hours, I have a professional background that gives me a better knowledge of oxygen systems and cardiopulmonary physiology. If you were an accountant, I wouldn't question you on tax law as it relates to aircraft, I ask for the same courtesy based on my background outside of aviation that happens to be cross-applicable.

But then again, if you can't rebuke the data on its merits, attack the person right?

 

[kyleb]

That's odd. I don't recall ever filling out that information....then again my accounts on there and on a couple of other forums did get "hacked" a year or so ago (by an ex-girlfriend on whose computer I used to access those forums) so that might explain where the bad info came from. I had to redo the information on my profiles and may have just overlooked the one over there. Thanks for bringing it to my attention.

That said, even if I am going along as a right front seat "passenger" (at least in the eyes of the FAA since I'm not PIC and it's not a dual pilot mandated aircraft), most of my flying has either been below 10,000 feet or in a pressurized aircraft (usually just handling the radios etc as the right seater). Simple enough?




Regardless if I am a low time pilot or one with 20,000 hours, I have a professional background that gives me a better knowledge of oxygen systems and cardiopulmonary physiology. If you were an accountant, I wouldn't question you on tax law as it relates to aircraft, I ask for the same courtesy based on my background outside of aviation that happens to be cross-applicable.

But then again, if you can't rebuke the data on its merits, attack the person right?

Attack? Naah, I made logical conclusions based on the data you presented here and elsewhere. I noticed that you didn't dispute anything I wrote.

By the way, the idea that your ex-girlfriend hacked your EAA account and populated your profile with erroneous data is priceless. Zoomlike, in fact.

 

[SteveinIndy]

I noticed that you didn't dispute anything I wrote.

You have your mind made up about me and your perception of superiority to me in all things aviation-related so why should I repeat the correct information? I figure I shouldn't give you anymore than basic courtesy because it will do no good to do more. So I will simply provide citations whenever I make a statement related to science, physiology, etc so that you know why I have the opinions I do. Most of them do not stem from flight experience because I do not believe in relying solely upon anecdotal evidence.

By the way, the idea that your ex-girlfriend hacked your EAA account and populated your profile with erroneous data is priceless. Zoomlike, in fact.

Welcome to the world of having a crazy stalker who knows how important aviation is to me because of my current line of work. On another forum there were some really nasty comments about how I "like little boys" and am an anti-Semite (which is absolutely hilarious given that I am Jewish) etc.

Then there was the stuff that she did in person including physical violence and threats against me and my current fiancee. Suffice to say that it took a restraining order against her to make it stop. I can state that dating someone who turns out to have schizotypal personality disorder can be fun at first but in the end it gets really, really ugly.

 

[SCCutler]

Did she eat your bunny?

 

[jesse]

How about we just focus on the topic and the obvious knowledge of the people posting instead of going on some witch hunt to pull stupid data off some website that nobody cares about putting accurate data into in the first place.

I've spent a few hours flying in an airplane and can tell you that Steve obviously knows a hell of a lot more about this subject than I do.

 

[SteveinIndy]

Did she eat your bunny?

No, but it took two sheriff's deputies to get my parrot back from her. But that is a topic for either a PM or another thread.

 

[Tom-D]

Oh, How I hate pressure breathing, BTDT, and no more.

There is no place I need to go, that requires O2.

 

[peter-h]

I often wonder if there might be a benefit to using an oxygen separator (molecular sieve or otherwise) with a higher throughput and just dumping the O2 in the cabin to raise the oxygen level for the whole thing. The nitrogen could even be used to inert the fuel system if one was so inclined.

You are screwed by the law of partial gas pressures.
Let's say the overall air pressure is 8psi (around FL180). 21% of this is due to oxygen, so the oxygen pressure is 21% of 8 i.e. 2psi.
If you now start pumping oxygen into the cockpit, and do enough of it to compensate for the reduced air pressure, and let's say you need to increase the oxygen % from 21% to 40% to achieve the same blood o2 % (sounds plausible, since the air pressure has halved), you will now have oxygen pressure in the cockpit of 40% of 8psi e.g. 3.2psi.
But the outside oxygen pressure is 2psi so you will have 1.2psi of oxygen trying to escape, and that is a huge pressure differential. The oxygen will be escaping rapidly through every orifice. It will push its way out through the heating / ventilating air inlets at a rapid flow rate, plus past the control linkages etc.
The cockpit won't actually be "pressurised" (much) because, similarly, the partial pressure of nitrogen (which is being depleted inside the cockpit) will work the other way round and the stuff will be trying to push its way in from the outside.
It's not going to work. You would need a huge gas output, to pressurise the cockpit by 1.2psi with oxygen, given all the leaks.
i with oxygen, given all the leaks.

 

[SCCutler]

You are screwed by the law of partial gas pressures.
Let's say the overall air pressure is 8psi (around FL180). 21% of this is due to oxygen, so the oxygen pressure is 21% of 8 i.e. 2psi.
If you now start pumping oxygen into the cockpit, and do enough of it to compensate for the reduced air pressure, and let's say you need to increase the oxygen % from 21% to 40% to achieve the same blood o2 % (sounds plausible, since the air pressure has halved), you will now have oxygen pressure in the cockpit of 40% of 8psi e.g. 3.2psi.
But the outside oxygen pressure is 2psi so you will have 1.2psi of oxygen trying to escape, and that is a huge pressure differential. The oxygen will be escaping rapidly through every orifice. It will push its way out through the heating / ventilating air inlets at a rapid flow rate, plus past the control linkages etc.
The cockpit won't actually be "pressurised" (much) because, similarly, the partial pressure of nitrogen (which is being depleted inside the cockpit) will work the other way round and the stuff will be trying to push its way in from the outside.
It's not going to work. You would need a huge gas output, to pressurise the cockpit by 1.2psi with oxygen, given all the leaks.
i with oxygen, given all the leaks.

Would not the o2 simply displace the N at the same pressure? I know you'd have plenty of leak-out, but you'd be continuously generating O2 to keep the concentration higher.

Probably stupid of me to ask, sorta like the junior high-school question, "If I connect a generator to a motor, won't I have a perpetual-motion machine?"

 

[peter-h]

AFAIK no.

In a mixture of gases (like air) each gas behaves as if it was the only gas present.

Imagine you have two equal volume tanks joined by a tube.

Tank 1 has oxygen at 10psi.
Tank 2 has nitrogen at 10psi.

What will happen is this:

Oxygen will push its way through the tube 1 -> 2 until both tanks have the same oxygen pressure (5psi).

Nitrogen will push its way through the tube 2 -> 1 until both tanks have the same nitrogen pressure (5psi).

After a while, both tanks will now have 5psi due to oxygen, and 5psi due to nitrogen. So the pressure in both will be 10psi again.

I hope I got that right

This is why it's hard to keep water vapour out of an unpressurised plane. You can pack the cockpit with silica gel which absorbs it, but the partial gas pressure of the vapour will just cause the stuff to flow back in. I have found one can achieve a 10 percentage point drop in RH with 1kg of silica gel (a large bag) but that's as far as one can get.

 

[SteveinIndy]

Would not the o2 simply displace the N at the same pressure? I know you'd have plenty of leak-out, but you'd be continuously generating O2 to keep the concentration higher.

That's the idea I was operating from. While Peter has a point, if you could pull enough O2 out, it might yield a slightly positive benefit.

Oxygen will push its way through the tube 1 -> 2 until both tanks have the same oxygen pressure (5psi).

Nitrogen will push its way through the tube 2 -> 1 until both tanks have the same nitrogen pressure (5psi).

If you're have a positive pressure coming out of the separator (as happens with most of them, since a home oxygen concentrator that many chronic lung patients use have this as the center of their "guts"), it should minimize that.

 

[peter-h]

You would need a very high oxygen generation rate to achieve 1.2psi of oxygen, given all the leaks.

Much much more than 3 litres/minute.

Also you would need a customised oxygen generator, which vents the nitrogen outside the cockpit. The present medical models vent it out of the box i.e. straight back into the cockpit, so they wouldn't do anything useful at all

My example of two tanks is just the law of partial gas pressures.

 

[denverpilot]

My personal practice is to wear a cannula at 2-4 L/min if above 6,000 ft (which is about where my SpO2 drops to 94 or 95% as it is with most of us "flat landers" who aren't physiologically adapted to living at altitudes) and a mask at 6-8 L/min above 10,000 ft in the rare cases I go that high in an unpressurized aircraft. This is based on what I know about high altitude physiology and experience with oxygen systems from over ten years as an EMS provider and a respiratory therapist.

Interesting numbers. I'm sitting at 98% here on my couch at 5880', give or take a few feet.

 

[SteveinIndy]

Interesting numbers. I'm sitting at 98% here on my couch at 5880', give or take a few feet.

Welcome to the wonders of physiological acclimation to altitude.

You would need a very high oxygen generation rate to achieve 1.2psi of oxygen, given all the leaks.

It's just my take (thinking with a homebuilder's mindset) but even in an unpressurized aircraft, it makes sense to minimize the leaks both during the build or maintenance if for nothing else than reduction of noise and making it easier to maintain a nice even cabin temperature. Inflatable door seals would go a long way to minimizing leaks.

Also you would need a customised oxygen generator, which vents the nitrogen outside the cockpit. The present medical models vent it out of the box i.e. straight back into the cockpit, so they wouldn't do anything useful at al

That's not a problem a length of tubing wouldn't alleviate. Either you could place the separator/concentrator (it's not a generator really...most of them are just a molecular sieve) outside of the cockpit and route the oxygen to the cockpit via the tubing or you place the unit in the cockpit and route the nitrogen outside. No need to get fancy or expensive honestly.

As for the volume required, some of the off-the-shelf medical versions can put out more than 25 L/minute (because they have a built-in reservoir system that is continuously refilled). I have seen an under ten pound industrial one that cranks out 300+ L/minute but I don't believe it would work because of the electrical requirements. Of course, these numbers are measured at sea level or something similar. I don't know what they would do at altitude but the information is presented simply for the sake of engineering discussion.