PO2 on Air

[DepthCharge]

1. Partial pressure of Oxygen has no effect on deco obligation.

2. If by this you mean simply switching your dive computer settings to 1.6 ppO2 then again it has no effect on deco obligations.

3. It doesn't within the range of recreational diving (not exceeding the depth of 40m/130ft).

This is only true for air.

 

[Tortuga68]

This is only true for air.

Please elaborate?

 

[DA Aquamaster]

Please elaborate?

What he is getting at is that by selecting a nitrox mix that will give you a PO2 of 1.4 at depth, you decrease the deco obligation by decreasing the percentage and partial pressure of the nitrogen at depth. Similarly, with planned decompression a PO2 of 1.6 is used to further accellerate off gassing of nitrogen.

It's one of the major reasons for using nitrox.

Or looking at it another way, if the percentage of O2 is constant (say 21%) the only way to increease the PO2 is to increase the depth with a PO2 of 1.4 being reached at 187 ft. (and a PO2 of 1.6 reached at 218 ft). So in effect, at 187 ft a 21% oxygen mix would give you the maximum percentage of O2 you could tolerate from an oxygen toxcity persepcetive on a working portion of the dive and it would in turn also give you the minmum decompression at that depth. Air however contains too much nitrogen and the resulting nitrogen narcosis is excessive so you would replace some of the nitrogen with a less narcotic gas like helium. That however also adds complications as helium is a "faster" gas that on gasses and offgasses at a different rate than nitrogen so it requires a different decompression schedule. But the 21% O2 percentage is still optimal for a PO2 of 1.4 at that depth.

Now you could make it really confusing by arguing the PO2 should be limited to 1.2 due to the decreased nitrogen in the trimix and the overall CNS clock issues when you consider the dive and accellerated deco at a max PO2 of 1.6 but that is even further beyond the scope of the question.

One of the advantages offerred by closed circuit rebreathers is that they allow you to continuously adjust the PO2 to keep it at a particular set point (1.2 or so) more or less regardless of depth to limit the decompression obligation compared to open circuit scuba where the PO2 can only be optimized for the maximum depth of the dive.

OK, I know this is the internet and we all can be who we want to be but I just realized something. Not trying to flame or bash the OP but he claims to have done 200 and 499 dives and he is asking a question like this? I find this really strange.

I am confident that a large number of recreational divers have little or no grasp of deco theory, partial pressure or math as it relates to gas laws in general. Today's OW courses just don't cover it to any great degree.

 

[mislav]

This is only true for air.

OP specifically asked about diving on air. So yes, please elaborate.

What he is getting at is that by selecting a nitrox mix that will give you a PO2 of 1.4 at depth, you decrease the deco obligation by decreasing the percentage and partial pressure of the nitrogen at depth. Similarly, with planned decompression a PO2 of 1.6 is used to further accellerate off gassing of nitrogen.

Emphasis added by me as this is not what OP has asked about.

 

[DepthCharge]

OP specifically asked about diving on air. So yes, please elaborate.


Emphasis added by me as this is not what OP has asked about.

Just pointing out that the statements AS WRITTEN are a bit misleading. You stated that PO2 has no effect on deco, you didn't specify on air. Aquamaster beat me to the response