Ron Lee

Ron Lee

Well Known Member
On a recent trip I climbed to 12,500' and allowed time for my system to "equalize" to ambient oxygen levels. I then put on the pulse oximeter and it read 90%.

I started the oxygen and within one to two minutes (not timed) the reading was 93% and by two to three minutes it was 96%. I stopped at that point since it was clear that using supplemental oxygen quickly raised my internal oxygen level.

I also checked it while at sea level. My memory may be off but I think it read 98%.
 
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Ron,

I see very similar (nearly identical) numbers on X-Cs in that altitude range, especially when its warm. Since it was recent (and probably warm or hot outside), your Density Altitude (DA) was probably a few thousand feet higher than 12.5, and I think that has an impact in lower O2 sats. O2 is key, and fully worth the expense, IMHO!

To add another data point, I've also found that if I go to 17.5 in the summer (DA's up above 19-20K), my low P O2 system and cannula's don't quite hack it, and my sats start to drop.

Good data Ron!

Cheers,
Bob
 
Bob, excellent point. I should have used density altitude if I understand your comment correctly. Yes it was warm. Perhaps 55 F at 12,500' MSL.

So my stated O2 blood level values are of limited value. Still, this was my first assessment of how long it took to change. It is quick.
 
Don't mean to start an argument, but is O2 saturation really dependent on density altitude? I would've thought pressure altitude is the controlling factor, since the body responds to the partial pressure of O2 in the atmosphere for uptake.
 
Ron Lee said:
Bob, excellent point. I should have used density altitude if I understand your comment correctly. Yes it was warm. Perhaps 55 F at 12,500' MSL.

So my stated O2 blood level values are of limited value. Still, this was my first assessment of how long it took to change. It is quick.
Click to expand...

Ron, I think that's true (about DA), but Kurt (krw) brings up a point worth examining. I don't want to push a theory that may not be fully true, so maybe the discussion below will generate some more knowledge from the field.

However, your numbers are certainly of value in similar conditions, which are prevalent this time of year.

As you said, the gist of it is, lower sats, and then hypoxia, can creep up pretty fast.

krw5927 said:
Don't mean to start an argument, but is O2 saturation really dependent on density altitude? I would've thought pressure altitude is the controlling factor, since the body responds to the partial pressure of O2 in the atmosphere for uptake.
Click to expand...

Honestly, I'm making an assumption (ooops, an educated...sort-of...guess, that is) on the effect of DA. Seems that less dense air would have a lower partial pressure of O2, though that may not be true. Perhaps there is just a lower concentration due to the lower density, but that's my theory...I can't quote the medical aspect of it, and I think its been batted about here before. I've just been erring on the side of being conservative, and start using O2 at a lower indicated altitude in the summer.

AFAIK, the regs don't call for supp O2 above specified DAs, they use indicated altitudes, so I could be totally out to lunch. Then again, the regs may have been written before all the cool EFISs with DA readouts came along! ;)

So for full disclosure, I'm definitely not an expert on this topic, and perhaps an RT (Webb, you out there?) or a Doc among us will have the definitive answer on the medical aspect. I'd like to learn more (off to Google it... :))

Good discussion, either way!

Cheers,
Bob
 
98.6 Degrees

Good point about the regs simply breaking it down to pressure altitude. But in my mind anyway partial pressure and density don't necessarily have to be related.

Rather, I was just thinking that since density altitude is pressure altitude adjusted for non-standard temperature, and since every breath you take rapidly assumes the standard body temperature of 98.6 degrees F, that the beginning density altitude of a particular breath wouldn't really matter. Maybe it does, maybe it doesn't, but your idea of being conservative by using O2 at lower pressure altitudes because density altitude is creeping up will do nothing but good. Heck, use it all the time if that's what tickles your fancy! I can't wait to wear cannulas because that'll mean my 9A is flying! (That seems so far away right now...)
 
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My Aerox system has individual needle valves for each person. Here's a link to them:

http://www.aerox.com/retrofit.html

This lets me adjust the flow rate to meet the need as I change altitude. I use an oxymeter to gauge it. With these, I can obtain good blood oxygen readings up to 18,000 feet. It definitely takes some oxygen flow there, though.

Dave
 
Well it appears that this has been debated quite often. Google produced lots of links to articles and (other) forum debates that discussed the same issue.

Verdict: mixed reviews...another never ending debate perhaps.

One of the most compelling was the article here: http://www.dr-amy.com/rich/oxygen/

Written by a doctor/pilot, it says its about PA, not DA. Refers to lungs being at the same DA for a given PA, regardless of the OAT, due to the fairly constant 98F/37C temperature in the lungs. (as you said Kurt!)

Of course, a few clicks later, came this: http://www.flightsafetycounselor.com/pilotphys.htm

Which says to take DA into account.

Saw plenty of arguments on both sides, may look farther. I'm leaning to accepting the DA argument (still open to learn more), but will exercise due caution, since I live at 5K and am up at 10-12K or more just get over to play with my formation buds. Have bottle, will travel! ;)

From a practical standpoint, a pulse-ox is the best guage to look at, since (from the articles) so many factors can influence your oxygen transport capacity...age, health, weight, fitness, smoker, what/how much you ate at the $100 burger stop, the Cee-Gar you shared with buds at the hangar...list goes on. But it appears an altitude that was a breeze one day, may be more arduous the next, depending on personal physical factors (caveat: that's my interpretation).

The meters are pretty inexpensive, and I use mine often. One more safety tool in an airplane with perfomance like ours! I think that's what Ron was gettin' at in the first place!

So go get that 9A flyin' Kurt, so you can test out the theories...and other fun stuff!! :D

Cheers,
Bob
 
Partial pressure of oxygen will decrease with altitude, just like every other gas. The fraction of oxygen is another matter. The fraction of oxygen (nominally ~20%) will decrease with altitude because oxygen is slightly heavier than nitrogen (which is most of the rest of the atmosphere. However, that decrease is down in the noise compared to the partial pressure decrease.

greg
 
Greg, as you state, the percentage of O2 is not likely a factor below 18,000'.

What I still do not know is whether the amount of O2 molecules (per breath) is a function of density altitude. Seems like it should be.
 
Ron,

It should be. The number of molecules will depend on the density of the air you are breathing. That density is a function of altitude and temperature, thus it is a function of density altitude. From basic chemistry/physics:

PV = nRT (pressure x volume = number of molecules x gas constant x temp)

therefore:

n = PV/RT

if one assumes that each breath is a constant volume, then the V and R terms don't change, and the number of molecules is simply a function of pressure and an inverse function of temperature. Since pressure is related to altitude (and weather, but in a more minor way), increasing pressure (lower altitude) will give you more oxygen. Similarly, decreasing temperature will give you more oxygen. These combined are effectively density altitude.

So, I haven't really told you anything more than you know already - oxygen in the air is a function of density altitude.

cheers,
greg
 
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