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.