PO2 limits...

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SkubaJim:
If you are breathing plain air, the O2 content is 21% or .21.
At two atmospheres (33 feet), you double the pressure to .42
At three atmospheres (66 feet), you add another .21 to it and get .63
This is your PO2 at that depth on plain air.

You can see if you are using 32% nitrox, you would have to use .32 instead of .21 in your calculations. You would reach the safe maximum PO2 of 1.40 at a depth a lot sooner than if you were on air.

Going to a 36% mix would put you at a PO2 of .72 at only 33 feet. (.36x2) Hope that math is right, I have had a glass of wine. :)

Anyway, as your depth increases, your mix (% of O2) becomes more critical in your calculations. 1.6 is absolute max and 1.4 is acceptable.

What kind of wine?

Have you ever tried "retsina"?
 
nereas:
I never said it was "universal." You may need new glasses.

Dude, you can't read at all. Did I say you said it was universal? Just wanted to put that info out for those who might not know of other agencies not recommending 1.4. :shakehead:
 
jtoorish:
1.4 would be a working PO2, but a PO2 of 1.6 is not unheard of for a resting dive, where a diver is not engaged in strenuous activity. A PO2 of 0.16 to 1.6 will sustain life. As has been mentioned, these numbers are not absolute.

I would, however, say a PO2 of between 1.1 and 1.3 is safer.

Jeff

Precisely right, I completely agree. And that is why I gave him/her a methodology for computing various limits, as well as fresh water in addition to seawater.
 
Charlie99:
To be more precise, a single tank diver that stays below 1.4ata ppO2 (partial pressure of oxygen less than 1.4 atmospheres absolute) is very unlikely to exceed the maximum recommended exposure time at that pressure --- 150 minutes per day using the NOAA tracking method.

Many divers erroneously think that the limit is merely a ppO2 limit. In reality, the limit is a combination of ppO2 and time.

For various partial pressures of oxygen, there are two exposure time limits. One is related to effects on the central nervous system (CNS toxicity, aka oxtox). Exceeding these limits can lead to convulsions, which in turn may lead to drowning.

The other limit is a longer term damage to the lung which reduces breathing capacity. Also sometimes called whole body toxicity, this is not normally a consideration for recreational divers.

These limits, and procedures for measuring %O2 in a tank, and calculating the depths associated with the ppO2 limits for various %O2 mixes are all covered in a nitrox class.

Charlie has introduced, and rightly so, advanced issues, such as, what really is safe? And what is not? And how long does it take before the piper comes calling to collect the bill?

For technical training, the issue of exposure time arises as well, particularly as you increase the PO2 for extended deco times.

But I seriously doubt our O/P (Original Poster) is a technical diver. So we would only confuse him/her with such further elaboration.

Is 1.4 safe?

Is 1.5 safe?

Is 1.6 safe?

Is 2.0 safe?

Is 3.0 safe?

At what level could you expect an instantaneous hit (convusion)?

And how long at other depths and pressures would you need to be exposed before a hit (convulsion) occurred?

I wonder about those esoteric questions as I program my deco software to keep me around 1.2 ATAs PO2 during my bottom time, and then run deco plans to ensure my cumulative exposure during deco does not exceed the red-line for the deco software (V-Planner).

I have a strong feeling all this theory would be lost upon our O/P however.

As a separate matter, some divers erroneously think that the limit is a combination of ppO2 and time. For recreational purposes, this is completely fallacious. Your surface intervals will clear out your oxygen clock. And therefore only a technical diver need seriously consider the effect of exposure time on PO2 limits.

1.4 is simply a convention. We need to draw the line somewhere for NDL divers.
 
nereas:
Charlie has introduced, and rightly so, advanced issues, such as, what really is safe? And what is not? And how long does it take before the piper comes calling to collect the bill?
I routinely point out the CNS toxicity limit is both TIME and PPO2 not because it is an advanced issue, but that it is very fundamental to understanding the limits.

It's not all that advanced. It's a standard time-dose combination sort of thing, much like the depth & time combinations for NDL. To just say 1.4ata and ignore time makes about as much sense as saying that one should stay above 50' in order to not get bent. (Or pick some other depth, based upon SAC and tank size assumptions).

For any ppO2, NOAA has an associated time limit. The limit for 1.4ppO2 is 150 minutes -- more than a typical recreational dive. The NOAA limit for 1.6ppO2 is 45 minutes ---- which isn't all that difficult to reach. Limiting ppO2 to 1.4ata max will limit your CNS exposure to an acceptable limit, but one should understand that it's a simplification.

Saying that GUE has a limit of 1.2ata is a similar simplification. The NOAA limit at 1.2ata is 210 minutes --- if you are diving doubles plus stages, you can get there on some extreme dives, and then going to 1.6ppO2 afterwards is just cranking up the CNS clock a bit further.

A more advanced topic, but still very easy to understand is the decay times associated with oxtox effects. The NOAA table calculation method is rather strange --- you accumulate all CNS clock % for an entire 24 hour period, and then after 24 hours is instantly disappears. Many dive computers use a different model, where there is a 90 minute (most computers) or 60 minute (Suunto computers) halftime assumed. In other words, were you to do a dive to 1.4ata for 150 minutes, you would have run your CNS clock to the max limit. After a 90 minutes surface interval though, the computer would assume that your CNS loading is now 50% of max. Very similar to the nitrogen calculations for a 90 minute halftime compartment.

===============

Thinking of the MOD as a rock hard depth beyond which you will instantaneously go into convlulsions causes many divers to think that they must abandon their buddies if for some reason their buddy strays below a 1.4MOD.

Something that any nitrox diver should consider is what ppO2 they are willing to go to in order to save their buddy. The extraordinary NOAA limits allow more than enough time at 2.0ata ppO2 to go save a buddy having problems below the classic 1.4ata MOD.
 
Charlie99:
I routinely point out the CNS toxicity limit is both TIME and PPO2 not because it is an advanced issue, but that it is very fundamental to understanding the limits.

It's not all that advanced. It's a standard time-dose combination sort of thing, much like the depth & time combinations for NDL. To just say 1.4ata and ignore time makes about as much sense as saying that one should stay above 50' in order to not get bent. (Or pick some other depth, based upon SAC and tank size assumptions).

For any ppO2, NOAA has an associated time limit. The limit for 1.4ppO2 is 150 minutes -- more than a typical recreational dive. The NOAA limit for 1.6ppO2 is 45 minutes ---- which isn't all that difficult to reach. Limiting ppO2 to 1.4ata max will limit your CNS exposure to an acceptable limit, but one should understand that it's a simplification.

===============

....

150 minutes, Charlie.

Do you know how much nitrox that would require? [I get 350 cubic feet. That is more than my twin-130s would hold.]

What is your definition of technical diving, I wonder?
 
nereas:
As a separate matter, some divers erroneously think that the limit is a combination of ppO2 and time. For recreational purposes, this is completely fallacious.
We will just have to agree to disagree. As I noted above (look at the very first sentence of post #3 in this thread), one can stay safe simply by limiting the ppO2 and making the assumption that one will not exceed the time limits in normal diving, but the limits are indeed a combination of time and ppO2.

My main objection to the oversimplification is that too many divers then think of the ppO2 limit as a hard, instantaneous limit, and are unwilling to exceed that limit in an emergency.
 
Charlie99:
We will just have to agree to disagree. As I noted above, one can stay safe simply by limiting the ppO2 and making the assumption that one will not exceed the time limits in normal diving, but the limits are indeed a combination of time and ppO2.

But you would need 350 cu ft of nitrox in your tank(s), Charlie.

I doubt that the O/P is going to strap on twin-130s together with three or more 40 cu ft deco bottles to have 350 cu ft of nitrox with him/her??
 
https://www.shearwater.com/products/perdix-ai/

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