Hypoxic mixes and density

So I'm sure this is in a hypoxic trimix course, but I haven't taken one yet, and I cant find an answer online, but, does density matter in O2 concentration for hypoxic mixes or just ppO2 for respiration? I didn't know if you could have a really low fraction of O2 (and density of O2) and a really high ppO2. I realize that pp is related to the fraction of the gas so the oxygen would diffuse. I just didn't know if the # of atoms could get low enough (at a low density) where the pp would be deceptive in giving a different theoretical result.
Thanks!

I'm a little fuzzy on exactly what you're asking, but I'll do my best to answer what I think you're getting at.

Simply put, density increases as your depth increases, as does the ppo2.

Naturally, trimix is less dense then air/nitrox to begin with (a full al80 of 32% nitrox sinks and a full al80 of 15/55 trimix floats). The lowered gas density of trimix (compared to nitrogen based gases at a given depth) is one of the good things about it. Easier to breath and less co2 retention.

The partial pressure of oxygen increases with depth the same way it does with air or nitrox. In addition to the narcotic advantaged of trimix, lowing the o2 content helps keep us safe at deeper depths. As an example, air has a ppo2 of 1.61 at 220ft. For many, that ppo2 is too high. Compare that to a 15% oxygen mix, and the ppo2 drops to a much more reasonable 1.15. At 320ft, a 15% oxygen mix crosses the 1.6ppo2 mark (I do NOT recommend taking 15% oxygen to 320ft).

Fritz01:

So I'm sure this is in a hypoxic trimix course, but I haven't taken one yet, and I cant find an answer online, but, does density matter in O2 concentration for hypoxic mixes or just ppO2 for respiration?

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Can you ask that same question a different way?

Fritz01, I have the impression you are mixing the terms "density" with "gas fraction" in your post. That is, when you refer to density, you do not mean [mass]/[volume], but actually [molecules of a certain gas]/[total number of molecules].

So, if I understand you correctly, I can say that the fraction of oxygen is not the relevant measure to determine if a gas can sustain life. Mixes with low o2%, say 4%, are used in deep dives. What is relevant is the ppO2, which does set both a floor and a ceiling of oxygen content in a mix for a given ambient pressure.

on an atomic level there may well be (is) a theoretical difference between the f02 of the gas in the cylinder and the f02 of the gas at ambient pressure.

the combined gas laws do not really stand up at low temps and high pressures .

as far as most diving goes the fraction of the various gases in the tank can be assumed to be a constant.

if the f02 when multiplied by the ambient falls between approx 0.18 and 1.4 then a diver should have enough 02 to remain conscious as long as he doesn't exeed the toxicity limits.

I didn't know if you could have a really low fraction of O2 (and density of O2) and a really high ppO2.

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The answer to this question is absolutely yes. ppO2 is determined by multiplying the percentage of O2 by the total ATA at the depth where you are. If you took 10/70 to 10 ATA, you'd have a ppO2 of 1.0. At 20 ATA (obviously, you aren't going there!) it would be 2.0.

But you're conflating partial pressures with density. Density is the mass present in a given volume, and it's determined by all the gases present in the mixture. There is no separate "density of O2" in the way that there is a partial pressure of O2. Helium-containing gases are less dense than air, because helium has a lower mass than nitrogen does, so when you have the same NUMBER of atoms in a volume, you have much less MASS.

Partial pressure is a different concept, which has to do with the fact that, in chemical processes, gases behave as though they are alone. For example, if you are breathing a 10/70 mix on land (don't do that!), oxygen will diffuse OUT of your body into your lungs, because the oxygen tension in the gas you are breathing is lower than that in your body. Helium will diffuse INTO your body, because the partial pressure in the lungs is higher than that in the tissues.

But density is a property of the gas as a whole.

Hey all! thanks for the responses I think I phrased the question poorly. I was wondering if there would ever be a case were an adequate ppO2 would not sustain life (assuming no other factors). My thought process was that (potentially) there could be a low enough fraction of O2 that while the oxygen could diffuse into the blood stream (given by fraction and depth), there would not be enough molecules to sustain life. Eg. a 1% mix at an incredible depth.
Note: I don't think you could ever get a partial pressure of an adequate value such that this would be the case, because the ppO2 indicates a higher concentration in the lungs, but I wasn't sure.

---------- Post added November 25th, 2013 at 05:27 PM ----------

Just to clarify: say for example you have 10 molecules at a ppO2 of 1.0 those 10 molecules would diffuse, but they would not sustain life. Unrealistic I know.

That's an interesting question. The VO2 of a resting or lightly exercising human is a known; if you got to a depth where the MASS of O2 was less than that, you would end up with hypoxic tissues despite an adequate pressure equivalent. I don't have enough time right now to look up the figures or do the calculations, but it would probably be a depth where total gas density rendered it unbreathable. I know people have been to 1000 feet on scuba, and people have worked in that general neighborhood in saturation habitats, so it's deeper than that.

Yes that is exactly the question I was asking!
Thank you, I heard of people going past 1000 and I didn't know if there was a potential problem with oxygen in hypoxic mixes at that depth.

The "partial pressure" is the chosen parameter to assess the adequacy of gases content in a mix because the partial pressure of a given gas indicates the rate at which molecules (or atoms, in the case of He) will go into or come out of solution. That means that for equal vapor pressure of gas in solution and equal partial pressure of gas out of solution, the number of molecules exchanged per unit time will be the same.

Of course, a high partial pressure of the gas will tend to make it dissolve rapidly, but a reduction in the number of molecules out of solution, if the total pressure remains constant, will mean a reduction of partial pressure of the gas. You cannot have a situation in which the ambient pressure remains constant, a high percentage of molecules of a gas goes into solution and this gas' partial pressure remains high.