Check my math regarding partial pressure.

[Texasguy]

Say that at the surface P02 is 0.21, that is 21% * 1ATA = 0.21

Say at 100feet ~ 3ATM, one breathing air is taking in 21% * 4ATA = 0.84 P02

Thus, going from this, for a human being, at surface to survive continuously, one needs to breath oxygen that at 1ATA is 0.21 PO2

Thus, at 100feet, to get minimal amount of oxygen in the mix for normal life I assume is still same 0.21 PO2 as on the surface? (correct?) Same amount of oxygen molecules as on the surface, same anywhere else, even under water at pressure.

Thus, at surface the mix would have 6% oxygen that would make it 0.21 PO2 at 100feet ~ 3ATM? (Calculated as .21PO2/4ATA = .06PO2/1ATA)

Therefore, would I be correct, that a mix of gas should have at least 6% oxygen in the tank to provide with minimum amount of oxygen for continuous life at 100 feet of water?

 

[boulderjohn]

I am not sure I understand your question, but it seems to me that you are asking if a percentage of oxygen that will not support life at the surface will effectively translate to a breathable mix at depth. If that is what you are asking, then that is certainly true. Your method finding that partial pressure at depth is also accurate.

Technical divers do this on deep dives. A diver going to 300 feet (about 10 atmospheres) will typically breathe a mix that has 10% oxygen, yielding a partial pressure of about 1.0 at that depth. The people going for the really deep dives will use a much lower percentage than that.

It does not have to be a partial pressure of .21 to support life, BTW. When I get in the water this afternoon here in Denver, I will have a tank with air in it, and my computer will flash me a big red warning sign that I am breathing a hypoxic mix until I get deep enough to overcome the effects of altitude. Here I breathe the equivalent of 0.18 air every day and somehow manage to stay alive.

 

[Jim Lapenta]

It's Denver. The ambient .03 percent cannabis vapor makes up for the lower O2

 

[redacted]

The minimum PPO2 to sustain life is about .16, with some tolerance for even lower levels.

 

[knotical]

The Navy and NOAA Diving manuals say (or at least used to say):

0.14 atm ppO2 – onset of hypoxic symptoms
0.08 atm – death

But I think 0.21 ppO2 is a reasonable minimum for diving (it allows for a bit of fudge factor).

---------- Post added August 12th, 2015 at 12:31 PM ----------

Update: now it says:

Air contains approximately 21 percent oxygen and provides an ample ppO2 of about 0.21 ata at the surface. A drop in ppO2 below 0.16 ata causes the onset of hypoxic symptoms. Most individuals become hypoxic to the point of helplessness at a ppO2 of 0.11 ata and unconscious at a ppO2 of 0.10 ata. Below this level, permanent brain damage and eventually death will occur. In diving, a lower percentage of oxygen will suffice as long as the total pressure is sufficient to maintain an adequate ppO2. For example, 5 percent oxygen gives a ppO2 of 0.20 ata for a diver at 100 fsw. On ascent, however, the diver would rapidly experience hypoxia if the oxygen percentage were not increased.

 

[tbone1004]

keep in mind that airliners are allowed to go to .75ata, so that's about PPO2 of .16 in line with everything else. That said you won't see anyone diving hypoxic mixes until they absolutely have to. This should be explained in your nitrox class though

 

[JohnnyC]

I'm not sure what you think the benefit would be of having a PO2 of .21 at 100ft. Is there something specific you are trying to accomplish by having a PO2 equivalent to surface? You still have to make up the other fractions of gas in the mix, so just breathing a hypoxic mix of 6% O2 (to get your .24PO2 at 100ft) means you're breathing 94% of other gasses. In the case of a simple O2/N2 mix, you'd be looking at a PN2 of 3.76, whereas something like a standard mix of 20/25 leaves you with a PN2 of 2.2, a pretty big difference. The deco implications would be quite real.

 

[PfcAJ]

Your understanding of the concept is correct. You can breath 6% at 100ft. However, 6% at the surface will put you to sleep. Don't do dat. A partial pressure below .16 is hypoxic and not safe to breath.

 

[JohnnyC]

Ran a couple numbers through MultiDeco just for giggles. First is normal air (30m/15m/21%), 100ft dive for 15 minutes, VPM +3. Second is the 6% hypoxic mix (30m/15m/6%). Red means the PO2 is too low to sustain life. As you can see, there's really no reason to dive with such a hypoxic mix. It quite literally gains you nothing other than an intellectual exercise in doing the math for PO2 calculations.

AIR
Dec to 30m (2) Air 15m/min descent.
Level 30m 13:00 (15) Air 0.84 ppO2, 30m ead
Asc to 6m (17) Air -9m/min ascent.
Stop at 6m 0:20 (18) Air 0.34 ppO2, 6m ead
Stop at 3m 2:00 (20) Air 0.27 ppO2, 3m ead
Surface (21) Air -3m/min ascent.

EAN6
Dec to 30m (2) Nitrox 6 15m/min descent.
Level 30m 13:00 (15) Nitrox 6 0.24 ppO2, 38m ead
Asc to 18m (16) Nitrox 6 -9m/min ascent.
Stop at 18m 0:40 (17) Nitrox 6 0.17 ppO2, 23m ead
Stop at 15m 3:00 (20) Nitrox 6 0.15 ppO2, 20m ead
Stop at 12m 4:00 (24) Nitrox 6 0.13 ppO2, 16m ead
Stop at 9m 5:00 (29) Nitrox 6 0.11 ppO2, 13m ead
Stop at 6m 9:00 (38) Nitrox 6 0.10 ppO2, 9m ead
Stop at 3m 18:00 (56) Nitrox 6 0.08 ppO2, 5m ead
Surface (57) Nitrox 6 -3m/min ascent.

 

[boulderjohn]

Ran a couple numbers through MultiDeco just for giggles. First is normal air (30m/15m/21%), 100ft dive for 15 minutes, VPM +3. Second is the 6% hypoxic mix (30m/15m/6%). Red means the PO2 is too low to sustain life. As you can see, there's really no reason to dive with such a hypoxic mix. It quite literally gains you nothing other than an intellectual exercise in doing the math for PO2 calculations.

The reason for diving hypoxic mixes is to avoid high PO2s that can lead to oxygen toxicity. In that case, you are going much deeper than 100 feet. In order to get the lower level of O2, you need to be doing something like adding helium to the mix.