Ppo2 & Theoretical Bottom

[yknot]

I was reading the course material for a nitrox course I signed up for and the section on partial pressures has me wondering. While it beyond the scope of a nitrox course, I am also aware that as you decend beyond rec depths you would decrease the O2% below what air contains (21% apx). At some point the O2% would be too low to sustain life, but I don't know what that level is. I believe that the current record depth on scuba is 1010'. At an O2% of 10% that would still be a PPO2 of 3.2 approx. What is the THEORETICAL max. depth for scuba (based on assured PPO2 induced O2 toxicity) while using the minimum O2% required for human respiration?

[King Kong Matt]

I believe (I am really not sure) that around .18 ppO2 is required to sustain life.

[Uncle Pug]

When diving to depths that require a very low % O2 to avoid an excessively high PO2 you would need a *travel gas* to get you to the point where you could begin to use your bottom gas.

So... the theroretical max depth for scuba is not necessarily dependent on PO2 exposure.

[Spectre]

Please note I'm not trimix certified

I believe a breathing gas becomes hypoxic at 16% O2; think of it as a PpO2 of .16. The difference between normoxic and hypoxic trimix is that hypoxic trimix is hypoxic at the surface... thus requiring 'travel gas' to get down to a depth where it is no longer hypoxic.

e.g. if you have a mix that has 8% O2, you need to travel to 2 ATA before it's safe to breathe. E.g. you can't switch to your back gas for deco if you have a problem with your travel bottle.

So... for a dive to 1010', if your planning on staying under 1.6, you'd want 5% O2, which gives you a PpO2 of 1.58 at 1010'.

However you'd need travel gasses to get you down to below 75' or so before you could breathe the gas.

Now one other note. There are theoretical O2 exposure time limits for PpO2 in excess of 1.6; otherwise no one would survive a dive below 220' on air.

To answer your question.. with enough bottles you can theoretically go as deep as you want [e.g. 1% is good down to over 5000', but you can't start breathing it until 500' or so].

Those who actually know this stuff can correct my numbers; but basically that's the situation AFAIK.

[Spectre]

Since you're reading the Nitrox materials, you're next question'll probably be about the limiting of nitrogen. Taking that 5% O2 mix, and trying to get an Equivalent Nitrogen Depth of 100' with your 1010' dive, you'd want 10% Nitrogen in the mix. So you'd end up with a mix of 5% O2 and 85% Helium.... otherwise you'll be too narced to remember to breathe! :)

[roturner]

Spectre once bubbled...
Please note I'm not trimix certified

<snip>

To answer your question.. with enough bottles you can theoretically go as deep as you want [e.g. 1% is good down to over 5000', but you can't start breathing it until 500' or so].


I think the pressure would get you before the O2 thing. It seems to me that your tank pressure has to be enough to overcome ambient pressure or your reg 1st stage will free flow. That's going to limit your working range. At 5000', even assuming big tanks that were magically still full, your reg would work for less than a minute before it started to free-flow, unable to close against the ambient pressure.

Maybe a more relevant question is the record. IIRC, the open circuit depth record is something like 304m set by a British guy with atomic powered balls. Some other testosterone junkie has said he wants to go to 350m this year. Deeper records have been set on other kinds of diving gear but not on open circuit. I'd be surprised if I don't see the 500m record in my life time.

R..

[Spectre]

roturner once bubbled...


I think the pressure would get you before the O2 thing

Oh... I didn't intend to imply that 5000' was even close to possible; I was just trying to make the point that gas partial pressures don't have much to do with a depth record....

[yknot]

Spectre once bubbled...
Since you're reading the Nitrox materials, you're next question'll probably be about the limiting of nitrogen. Taking that 5% O2 mix, and trying to get an Equivalent Nitrogen Depth of 100' with your 1010' dive, you'd want 10% Nitrogen in the mix. So you'd end up with a mix of 5% O2 and 85% Helium.... otherwise you'll be too narced to remember to breathe! :)


Out of curiosity, is the nitrogen even necessary? If with the above example could the Helium be 95% and the Nitrogen at 0? Also, would there be some time required to offgas the Helium?

[King Kong Matt]

edited...I thought my information was accurate but it was not in light of the specific example given...do not heed previous post (not that you would or that anyone does :) )

[Uncle Pug]

yknot once bubbled...
Out of curiosity, is the nitrogen even necessary?
Beyond ~400' some nitrogen is necessary to supress high pressure nervous syndrome.

See Dr. Deco's post here (http://www.scubaboard.com/showthread.php?s=&postid=157412&highlight=hpns#post157412)

[Divesherpa]

yknot once bubbled...



Out of curiosity, is the nitrogen even necessary? If with the above example could the Helium be 95% and the Nitrogen at 0? Also, would there be some time required to offgas the Helium?

High Pressure Nervous Syndrome becomes a problem if there is no nitrogen in the mix for major times at depth (saturation). The slight nitrogen buffer adds safety. And yes, Helium requires offgassing just like Nitrogen. Helium comes out more than 2.5 times faster, but Helium is also absorbed that much faster.
If you are really interested, you could do a lot of diving and get into a trimix class after a couple hundred dives.:D
Cheers and safe diving

[Divesherpa]

Looks like Pug and I posted at the same time. OOps