Discussion in 'Basic Scuba Discussions' started by jsado, Aug 30, 2007.
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.
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.
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).
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.
Nereas, Great explanation!
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.
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
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.
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.
I never said it was "universal." You may need new glasses.
What kind of wine?
Have you ever tried "retsina"?
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:
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.
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.
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.
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?
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.
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??
Separate names with a comma.