PO2 limits...

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Expat Floridian travelling in the Land of Eternal
# of dives
500 - 999
Can someone explain PO2 limits to me?

When you breathe underwater, the oxygen in your mix must not exceed a certain limit of pressure. If you exceed that limit, then you will experience a convulsion, and possibly lose consciousness. This is not good.

No one knows exactly what that limit is, however it has been generally agreed that 1.4 atmospheres-absolute is safe, and exceeding 1.4 ATAs should be avoided. This is the current most widely used acceptible PO2 limit for recreational scuba diving.

Air is a mix of gasses that includes 21% oxygen, 78% nitrogen, 0.9% argon, and 0.1% other trace gasses such as CO2, neon, etc.

If you divide 1.4 ATAs by 21% you get 6.7 ATAs.

You can check your math and multiply 6.7 ATAs by 21% which will give you 1.4 ATAs.

Therefore breathing air will not pose a PO2 problem for you as long as the ambient pressure at which you are breathing it is equal to or less than 6.7 ATAs. This is beyond your 130 ft recreational scuba limit, so with air it is something that you do not need to worry about.

Nitrox however is a different mix than air. EAN 32 for example is 32% oxygen.

If you divide 1.4 by 32% you get 4.375 ATAs.

You can check your math and multiply 4.375 ATAs by 32% which will give you 1.4 ATAs.

Now, with nitrox, you do need to worry about your depth, which results in increasing your ambient pressure, since 4.375 ATAs is within your 130 ft recreational limits.

On the surface, your ambient pressure is 1.0 ATAs. This is by definition.

At 33 feet depth of seawater, your ambient pressure is 2.0 ATAs. This is computed as follows:

1 + (33 feet depth / 33) = 2

At 66 feet, your ambient pressure is 3.0 ATAs computed as follows:

1 + (66 feet depth / 33) = 3

At 99 feet, your ambient pressure is 4.0 ATAs, computed as:

1 + (99 feet depth / 33) = 4

To determine exactly how deep 4.375 ATAs is, you must do the reverse of the above calculations:

Step 1: 4.375 - 1 = 3.375

Step 2: 3.375 x 33 ft = 111 ft.

This means that you should not dive deeper than 111 ft with EAN 32 in order to follow the 1.4 ATA limit rule.

You can check your math by computing the ambient pressure at 111 ft of seawater as follows:

1 + (111/33) = 4.37

So you see how it works, and why.

To do these calculations for fresh water lakes or quarries, you would use 34 ft instead of 33.

And to do them for a different agreed-upon limit such as 1.6 ATAs, you would replace 1.4 with 1.6.

I hope you are good at math! That is what nitrox requires.

In college physics and chemistry, the professor(s) will have explained during lecture that PO2 is expressed as a partial pressure determined by multiplying the fraction of a given gas within a mixture of gasses by the total pressure of the combined gas mix. That is what I have shown you how to do, iteratively.

Iterative learning is a guided discovery method of teaching that takes you through a number of steps over and over until you understand them.


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Silicon Valley, CA / New Bedford, MA / Kihei, Maui
# of dives
500 - 999
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.


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San Francisco, CA
Furthermore, 1.4 isn't a universal standard. Many tech divers will go above it for deco. The U.S. Navy is also more conservative, and uses 1.3. GUE teaches 1.2 (for cold diving).


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Naples, Florida
# of dives
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.

Jeff Toorish

Scuba Instructor
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North of Boston, South of Canada!
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.



Kimber/TekDiveGirl storyteller and memory keeper
ScubaBoard Sponsor
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San Diego, Ca.
# of dives
50 - 99
A point about higher PO2 for deco (I've heard 1.6 for deco) you are not moving, or moving very little when doing your stops


Scuba Instructor
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Branford, FL
# of dives
500 - 999
Hello jsado, just for fun, you can use the math above and determine why we can only dive to 218 feet using normal air (21% O2). We would exceed a safe po2 and convulse eventually. Sounds like you may be interested in taking a Nitrox class soon. Have fun if you do.


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Carol Stream, IL
# of dives
200 - 499
Let me try.
In any mix of gasses, each gas exerts a certain amount of pressure. Using air as an example, 20.9% of the mixture is Oxygen. 78% is Nitrogen, Argon, Helium and other trace gasses make up the remainder. As Oxygen makes up 20.9% of the mixture, it is responsible for 20.9% of the overall pressure of that mix. So, a tank of air at 100psi has 20.9% or 20.9psi comming from just the Oxygen. Surface air not in a container is at 14.7psi. of that 14.7psi, 20.9% or 3.07psi is Oxygen. Therefor, the PPO2 of air at the surface is .209. With me so far?

As you decend in a water collumn the pressure builds. At 33' or 2ATA you have doubled the pressure at the surface. Each breath you take is now double the volume it was at the surface. The amount of oxygen and Nitrogen in each breath is double what it was on the surface. Air not in a rigid container at this depth is now at 29.4psi, or double the 14.7 psi on ht surface. The Oxygen in that air has doubled as well so it now exerts 41.8% of the total pressure. (remember, we are now working with 200% of air since it doubled.)
Now we go deeper, to 6.7 ATA or 188'. If you multiply the 6.7ATA by the 20.9 (oxygen in air at the surface) you see we get 1.4PPO2. Each breath now contains 6.7 times more O2 than it did at the surface. That 1.4 is considered by some to be a max working PPO2 that is safe.

If we enrich that standard air with more O2 things change a bit. A standard mix is 32% Nitrox which means there is 32% O2, 67%Nitrogen and 1% other. At 2ATA the O2 is now exerting 64%.

At 4.37ATA or 111' our partial pressure of O2 is 1.4 (4.37ATA times the 32% O2 in the mix). Notice, since the ammount of O2 in the mix went up we reached a PP of O2 at a shallower depth.

Take a Nitrox course and it will all be more clear. I just wanted to make sure I could explain it. Hope that helped.

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