PO2 on Air

[QuarryDiver23]

Question in three parts that I have from several discussions at the LDS in understanding PO2 while diving air.

1. Does a PO2 of 1.4 diving AIR (21%) decrease/increase the amount of deco obligation you have at the end of a dive?

2. If you change the PO2 to 1.6 diving AIR (21%) and repeat the exact same dive will that increase the amount of deco obligation you will have at the end of the dive?

3. Does a PO2 come into play while diving AIR (21%)?

Thanks, I would appreciate any input.

 

[mislav]

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).

 

[QuarryDiver23]

Thanks. I think that I need to ask it this way for me to understand.

Is or is not the controlling gas for deco Nitrogen? On air = 21% oxygen 79% nitrogen.

Senario:
If you were on a dive and you were at 100ft and exceeded the NDL for 10 minutes - if your computer was set on a PPO2 of 1.6 would your deco time be more than if it was set on 1.4?

Or again it does not effect deco time.

Thanks again.

 

[knotical]

Recall that when breathing air, the partial pressure of oxygen at maximum recreational depth only slightly exceeds 1.0. The 1.4 and 1.6 numbers are simply ceilings that you will not even approach.

Without knowing the specifics of the algorithm in your computer, one cannot be certain - but setting it for 1.4 or 1.6 should have no impact on deco obligation.

Yes, when using air or nitrox, nitrogen rules decompression.

 

[TSandM]

I think you are confused a bit. The ppO2 setting on your computer has nothing to do with what you are breathing; it's simply the point at which it will sound an alarm.

Your ppO2 at any depth is a product of the percentage of oxygen in your mix (which is 21%, if you are diving air) and the depth you are at. If you are at 100 fsw (which is 4 ATA), your ppO2 on air is .84 ATA. It doesn't matter what you have set your computer at. To get a higher ppO2, you have to go deeper. At 130 feet, which is 5 ATA, your ppO2 is 1.0. Obviously, the deeper you go, the shorter the time you can stay there before you incur a decompression obligation. But that's not because of the ppO2. It's because at the same time the partial pressure of oxygen is going up, so is the partial pressure of nitrogen, so you're absorbing more.

You can see that, on air, within recreational depths, you are not going to push the ppO2 into the range which is worrisome for toxicity (which is variably defined, but definitely more than 1.2 ATA).

When you start using Nitrox, on the other hand, you CAN get up into those numbers. At 4 ATA (100 fsw), 32% Nitrox gives you a ppO2 of 1.28, which is getting up there, especially if you are working hard on the dive (swimming into current, for example). At the same time, you have a longer allowable time at that depth before incurring a decompression obligation, because the percentage of nitrogen is lower than it would have been on air.

What you set your computer for doesn't change what you're breathing, or change the amount of gas in your blood at any given depth. It simply tells the computer how high a level of oxygen you are willing to tolerate (or, to put it another way, how high a risk of oxygen toxicity you are willing to tolerate) before you want the computer to say something to you about it.

 

[QuarryDiver23]

Thanks for the information. This is just one of those things were all of the information is right infront of me and it was just not sinking in.

So, for this discussion as I understand it, the partial pressure pertains to the toxicity of O2 at a given depth. On the recreational side while diving air we are well within acceptable levels. Air does not become toxic until you reach a PPO2 of 1.6. Which would then explain the need for trimix.

Changing the setting of the PO2 on my computer does not effect my deco obligation. Understood!

Also then is it safe to say that nitrogen is a non-toxic gas the the system? But as the partial pressure of nitrogen increases for a given depth so does the absorption level into your system -which then equates to decompression obligation for a given depth + time.

Thanks.

 

[wedivebc]

For air to reach PO2 of 1.6 you would have to go to 218fsw. Nitrogen narcosis would be an issue long before that. Air is very impractical from a deco perspective so I would suggest if you are even thinking about planned decompression diving then start thinking about using nitrox.

Thanks for the information. This is just one of those things were all of the information is right infront of me and it was just not sinking in.

So, for this discussion as I understand it, the partial pressure pertains to the toxicity of O2 at a given depth. On the recreational side while diving air we are well within acceptable levels. Air does not become toxic until you reach a PPO2 of 1.6. Which would then explain the need for trimix.

Changing the setting of the PO2 on my computer does not effect my deco obligation. Understood!

Also then is it safe to say that nitrogen is a non-toxic gas the the system? But as the partial pressure of nitrogen increases for a given depth so does the absorption level into your system -which then equates to decompression obligation for a given depth + time.

Thanks.

 

[TSandM]

Nitrogen doesn't cause toxicity the way oxygen does, but nitrogen at high pressures is essentially an anesthetic gas, which is why you get nitrogen narcosis. But you are quite right that, at deeper depths, the higher pressure of nitrogen results in more nitrogen absorption into the body, which is why you can't stay at depth as long as you can in the shallows without incurring a decompression obligation. (I mean, we all incur decompression obligations on EVERY SINGLE DIVE, in the sense that you have to control your ascent rate to allow the nitrogen to get out of your body. But you don't have to stop and sit anywhere in the water for long periods of time, to exit without DCS symptoms.)

You might want to pull out your OW written materials and review the part on gas dynamics, because this should all be covered pretty well there, as far as nitrogen kinetics go. The oxygen toxicity stuff really doesn't get covered until you take a Nitrox class.

 

[crpntr133]

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.

 

[gcbryan]

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.

You can have 200 dives and not be Nitrox certified and not be conversant with the concept of partial pressures.

It's a little odd perhaps and even more odd is the fact that we haven't heard from BigTim in a while.